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gitdiffcore doc: mention new preliminary step for rename detection

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The last few patches have introduced a new preliminary step when rename
detection is on but both break detection and copy detection are off.
Document this new step.  While we're at it, add a testcase that checks
the new behavior as well.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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Elijah Newren 2021-02-14 07:51:50 +00:00 committed by Junio C Hamano
parent bd24aa2f97
commit 07c9a7fcb5
1 changed files with 20 additions and 0 deletions
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Documentation/gitdiffcore.txt
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@ -168,6 +168,26 @@ a similarity score different from the default of 50% by giving a
number after the "-M" or "-C" option (e.g. "-M8" to tell it to use
8/10 = 80%).
Note that when rename detection is on but both copy and break
detection are off, rename detection adds a preliminary step that first
checks if files are moved across directories while keeping their
filename the same. If there is a file added to a directory whose
contents is sufficiently similar to a file with the same name that got
deleted from a different directory, it will mark them as renames and
exclude them from the later quadratic step (the one that pairwise
compares all unmatched files to find the "best" matches, determined by
the highest content similarity). So, for example, if a deleted
docs/ext.txt and an added docs/config/ext.txt are similar enough, they
will be marked as a rename and prevent an added docs/ext.md that may
be even more similar to the deleted docs/ext.txt from being considered
as the rename destination in the later step. For this reason, the
preliminary "match same filename" step uses a bit higher threshold to
mark a file pair as a rename and stop considering other candidates for
better matches. At most, one comparison is done per file in this
preliminary pass; so if there are several remaining ext.txt files
throughout the directory hierarchy after exact rename detection, this
preliminary step will be skipped for those files.
Note. When the "-C" option is used with `--find-copies-harder`
option, 'git diff-{asterisk}' commands feed unmodified filepairs to
diffcore mechanism as well as modified ones. This lets the copy