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>

contrib/subtree/git-subtree.sh

@@ -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)