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Documentation: merge: add a section about fast-forward

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Novices sometimes find the behavior of 'git merge' in the
fast-forward case surprising.  Describe it thoroughly.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Thomas Rast <trast@student.ethz.ch>
This commit is contained in:
Jonathan Nieder 2010-01-23 03:45:33 -06:00 committed by Thomas Rast
parent 30f2bade84
commit 29280311f0
1 changed files with 18 additions and 13 deletions
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31
Documentation/git-merge.txt
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@ -86,25 +86,30 @@ would result from the merge already.)
If all named commits are already ancestors of `HEAD`, 'git merge'
will exit early with the message "Already up-to-date."
FAST-FORWARD MERGE
------------------
Often the current branch head is an ancestor of the named commit.
This is the most common case especially when invoked from 'git
pull': you are tracking an upstream repository, you have committed
no local changes, and now you want to update to a newer upstream
revision. In this case, a new commit is not needed to store the
combined history; instead, the `HEAD` (along with the index) is
updated to point at the named commit, without creating an extra
merge commit.
This behavior can be suppressed with the `--no-ff` option.
HOW MERGE WORKS
---------------
A merge is always between the current `HEAD` and one or more
commits (usually a branch head or tag).
Two kinds of merge can happen:
* `HEAD` is already contained in the merged commit. This is the
most common case especially when invoked from 'git pull':
you are tracking an upstream repository, have committed no local
changes and now you want to update to a newer upstream revision.
Your `HEAD` (and the index) is updated to point at the merged
commit, without creating an extra merge commit. This is
called "Fast-forward".
* Both the merged commit and `HEAD` are independent and must be
tied together by a merge commit that has both of them as its parents.
The rest of this section describes this "True merge" case.
Except in a fast-forward merge (see above), the branches to be
merged must be tied together by a merge commit that has both of them
as its parents.
The rest of this section describes this "True merge" case.
The chosen merge strategy merges the two commits into a single
new source tree.