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subtree: improve decision on merges kept in split 

When multiple identical parents are detected for a commit being considered
for copying, explicitly check whether one is the common merge base between
the commits. If so, the other commit can be used as the identical parent;
if not, a merge must be performed to maintain history.

In some situations two parents of a merge commit may appear to both have
identical subtree content with each other and the current commit. However,
those parents can potentially come from different commit graphs.

Previous behavior would simply select one of the identical parents to
serve as the replacement for this commit, based on the order in which they
were processed.

New behavior compares the merge base between the commits to determine if
a new merge commit is necessary to maintain history despite the identical
content.

Signed-off-by: Strain, Roger L <roger.strain@swri.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Strain, Roger L 2018-09-28 13:35:40 -05:00 committed by Junio C Hamano
parent 315a84f9aa
commit 68f8ff8151
1 changed files with 19 additions and 2 deletions
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21
contrib/subtree/git-subtree.sh
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@ -541,6 +541,7 @@ copy_or_skip () {
nonidentical=
p=
gotparents=
copycommit=
for parent in $newparents
do
ptree=$(toptree_for_commit $parent) || exit $?
@ -548,7 +549,24 @@ copy_or_skip () {
if test "$ptree" = "$tree"
then
# an identical parent could be used in place of this rev.
identical="$parent"
if test -n "$identical"
then
# if a previous identical parent was found, check whether
# one is already an ancestor of the other
mergebase=$(git merge-base $identical $parent)
if test "$identical" = "$mergebase"
then
# current identical commit is an ancestor of parent
identical="$parent"
elif test "$parent" != "$mergebase"
then
# no common history; commit must be copied
copycommit=1
fi
else
# first identical parent detected
identical="$parent"
fi
else
nonidentical="$parent"
fi
@ -571,7 +589,6 @@ copy_or_skip () {
fi
done
copycommit=
if test -n "$identical" && test -n "$nonidentical"
then
extras=$(git rev-list --count $identical..$nonidentical)