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272 lines
11 KiB
Text
272 lines
11 KiB
Text
Tweaking diff output
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====================
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June 2005
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Introduction
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------------
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The diff commands git-diff-index, git-diff-files, and git-diff-tree
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can be told to manipulate differences they find in
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unconventional ways before showing diff(1) output. The manipulation
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is collectively called "diffcore transformation". This short note
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describes what they are and how to use them to produce diff outputs
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that are easier to understand than the conventional kind.
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The chain of operation
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----------------------
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The git-diff-* family works by first comparing two sets of
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files:
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- git-diff-index compares contents of a "tree" object and the
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working directory (when '\--cached' flag is not used) or a
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"tree" object and the index file (when '\--cached' flag is
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used);
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- git-diff-files compares contents of the index file and the
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working directory;
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- git-diff-tree compares contents of two "tree" objects;
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In all of these cases, the commands themselves compare
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corresponding paths in the two sets of files. The result of
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comparison is passed from these commands to what is internally
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called "diffcore", in a format similar to what is output when
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the -p option is not used. E.g.
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------------------------------------------------
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in-place edit :100644 100644 bcd1234... 0123456... M file0
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create :000000 100644 0000000... 1234567... A file4
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delete :100644 000000 1234567... 0000000... D file5
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unmerged :000000 000000 0000000... 0000000... U file6
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------------------------------------------------
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The diffcore mechanism is fed a list of such comparison results
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(each of which is called "filepair", although at this point each
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of them talks about a single file), and transforms such a list
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into another list. There are currently 6 such transformations:
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- diffcore-pathspec
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- diffcore-break
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- diffcore-rename
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- diffcore-merge-broken
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- diffcore-pickaxe
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- diffcore-order
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These are applied in sequence. The set of filepairs git-diff-\*
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commands find are used as the input to diffcore-pathspec, and
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the output from diffcore-pathspec is used as the input to the
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next transformation. The final result is then passed to the
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output routine and generates either diff-raw format (see Output
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format sections of the manual for git-diff-\* commands) or
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diff-patch format.
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diffcore-pathspec: For Ignoring Files Outside Our Consideration
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---------------------------------------------------------------
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The first transformation in the chain is diffcore-pathspec, and
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is controlled by giving the pathname parameters to the
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git-diff-* commands on the command line. The pathspec is used
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to limit the world diff operates in. It removes the filepairs
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outside the specified set of pathnames. E.g. If the input set
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of filepairs included:
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------------------------------------------------
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:100644 100644 bcd1234... 0123456... M junkfile
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------------------------------------------------
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but the command invocation was "git-diff-files myfile", then the
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junkfile entry would be removed from the list because only "myfile"
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is under consideration.
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Implementation note. For performance reasons, git-diff-tree
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uses the pathname parameters on the command line to cull set of
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filepairs it feeds the diffcore mechanism itself, and does not
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use diffcore-pathspec, but the end result is the same.
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diffcore-break: For Splitting Up "Complete Rewrites"
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----------------------------------------------------
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The second transformation in the chain is diffcore-break, and is
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controlled by the -B option to the git-diff-* commands. This is
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used to detect a filepair that represents "complete rewrite" and
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break such filepair into two filepairs that represent delete and
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create. E.g. If the input contained this filepair:
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------------------------------------------------
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:100644 100644 bcd1234... 0123456... M file0
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------------------------------------------------
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and if it detects that the file "file0" is completely rewritten,
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it changes it to:
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------------------------------------------------
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:100644 000000 bcd1234... 0000000... D file0
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:000000 100644 0000000... 0123456... A file0
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------------------------------------------------
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For the purpose of breaking a filepair, diffcore-break examines
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the extent of changes between the contents of the files before
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and after modification (i.e. the contents that have "bcd1234..."
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and "0123456..." as their SHA1 content ID, in the above
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example). The amount of deletion of original contents and
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insertion of new material are added together, and if it exceeds
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the "break score", the filepair is broken into two. The break
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score defaults to 50% of the size of the smaller of the original
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and the result (i.e. if the edit shrinks the file, the size of
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the result is used; if the edit lengthens the file, the size of
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the original is used), and can be customized by giving a number
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after "-B" option (e.g. "-B75" to tell it to use 75%).
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diffcore-rename: For Detection Renames and Copies
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-------------------------------------------------
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This transformation is used to detect renames and copies, and is
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controlled by the -M option (to detect renames) and the -C option
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(to detect copies as well) to the git-diff-* commands. If the
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input contained these filepairs:
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------------------------------------------------
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:100644 000000 0123456... 0000000... D fileX
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:000000 100644 0000000... 0123456... A file0
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------------------------------------------------
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and the contents of the deleted file fileX is similar enough to
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the contents of the created file file0, then rename detection
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merges these filepairs and creates:
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------------------------------------------------
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:100644 100644 0123456... 0123456... R100 fileX file0
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------------------------------------------------
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When the "-C" option is used, the original contents of modified files,
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and deleted files (and also unmodified files, if the
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"\--find-copies-harder" option is used) are considered as candidates
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of the source files in rename/copy operation. If the input were like
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these filepairs, that talk about a modified file fileY and a newly
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created file file0:
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------------------------------------------------
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:100644 100644 0123456... 1234567... M fileY
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:000000 100644 0000000... bcd3456... A file0
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------------------------------------------------
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the original contents of fileY and the resulting contents of
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file0 are compared, and if they are similar enough, they are
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changed to:
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------------------------------------------------
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:100644 100644 0123456... 1234567... M fileY
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:100644 100644 0123456... bcd3456... C100 fileY file0
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------------------------------------------------
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In both rename and copy detection, the same "extent of changes"
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algorithm used in diffcore-break is used to determine if two
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files are "similar enough", and can be customized to use
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a similarity score different from the default of 50% by giving a
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number after the "-M" or "-C" option (e.g. "-M8" to tell it to use
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8/10 = 80%).
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Note. When the "-C" option is used with `\--find-copies-harder`
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option, git-diff-\* commands feed unmodified filepairs to
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diffcore mechanism as well as modified ones. This lets the copy
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detector consider unmodified files as copy source candidates at
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the expense of making it slower. Without `\--find-copies-harder`,
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git-diff-\* commands can detect copies only if the file that was
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copied happened to have been modified in the same changeset.
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diffcore-merge-broken: For Putting "Complete Rewrites" Back Together
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--------------------------------------------------------------------
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This transformation is used to merge filepairs broken by
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diffcore-break, and not transformed into rename/copy by
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diffcore-rename, back into a single modification. This always
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runs when diffcore-break is used.
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For the purpose of merging broken filepairs back, it uses a
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different "extent of changes" computation from the ones used by
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diffcore-break and diffcore-rename. It counts only the deletion
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from the original, and does not count insertion. If you removed
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only 10 lines from a 100-line document, even if you added 910
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new lines to make a new 1000-line document, you did not do a
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complete rewrite. diffcore-break breaks such a case in order to
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help diffcore-rename to consider such filepairs as candidate of
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rename/copy detection, but if filepairs broken that way were not
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matched with other filepairs to create rename/copy, then this
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transformation merges them back into the original
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"modification".
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The "extent of changes" parameter can be tweaked from the
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default 80% (that is, unless more than 80% of the original
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material is deleted, the broken pairs are merged back into a
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single modification) by giving a second number to -B option,
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like these:
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* -B50/60 (give 50% "break score" to diffcore-break, use 60%
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for diffcore-merge-broken).
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* -B/60 (the same as above, since diffcore-break defaults to 50%).
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Note that earlier implementation left a broken pair as a separate
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creation and deletion patches. This was an unnecessary hack and
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the latest implementation always merges all the broken pairs
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back into modifications, but the resulting patch output is
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formatted differently for easier review in case of such
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a complete rewrite by showing the entire contents of old version
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prefixed with '-', followed by the entire contents of new
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version prefixed with '+'.
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diffcore-pickaxe: For Detecting Addition/Deletion of Specified String
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---------------------------------------------------------------------
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This transformation is used to find filepairs that represent
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changes that touch a specified string, and is controlled by the
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-S option and the `\--pickaxe-all` option to the git-diff-*
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commands.
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When diffcore-pickaxe is in use, it checks if there are
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filepairs whose "original" side has the specified string and
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whose "result" side does not. Such a filepair represents "the
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string appeared in this changeset". It also checks for the
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opposite case that loses the specified string.
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When `\--pickaxe-all` is not in effect, diffcore-pickaxe leaves
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only such filepairs that touch the specified string in its
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output. When `\--pickaxe-all` is used, diffcore-pickaxe leaves all
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filepairs intact if there is such a filepair, or makes the
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output empty otherwise. The latter behaviour is designed to
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make reviewing of the changes in the context of the whole
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changeset easier.
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diffcore-order: For Sorting the Output Based on Filenames
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---------------------------------------------------------
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This is used to reorder the filepairs according to the user's
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(or project's) taste, and is controlled by the -O option to the
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git-diff-* commands.
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This takes a text file each of whose lines is a shell glob
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pattern. Filepairs that match a glob pattern on an earlier line
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in the file are output before ones that match a later line, and
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filepairs that do not match any glob pattern are output last.
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As an example, a typical orderfile for the core git probably
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would look like this:
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------------------------------------------------
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README
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Makefile
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Documentation
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*.h
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*.c
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t
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------------------------------------------------
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