rebase --preserve-merges: keep all merge commits including empty ones

Since 90e1818  (git-rebase: add keep_empty flag, 2012-04-20)
'git rebase --preserve-merges' fails to preserve empty merge commits
unless --keep-empty is also specified.  Merge commits should be
preserved in order to preserve the structure of the rebased graph,
even if the merge commit does not introduce changes to the parent.

Teach rebase not to drop merge commits only because they are empty.

A special case which is not handled by this change is for a merge commit
whose parents are now the same commit because all the previous different
parents have been dropped as a result of this rebase or some previous
operation.

Signed-off-by: Phil Hord <hordp@cisco.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--interactive.sh

@@ -175,6 +175,11 @@ is_empty_commit() {
 	test "$tree" = "$ptree"
 }
 
+is_merge_commit()
+{
+	git rev-parse --verify --quiet "$1"^2 >/dev/null 2>&1
+}
+
 # Run command with GIT_AUTHOR_NAME, GIT_AUTHOR_EMAIL, and
 # GIT_AUTHOR_DATE exported from the current environment.
 do_with_author () {
@@ -796,7 +801,7 @@ git rev-list $merges_option --pretty=oneline --abbrev-commit \
 while read -r shortsha1 rest
 do
 
-	if test -z "$keep_empty" && is_empty_commit $shortsha1
+	if test -z "$keep_empty" && is_empty_commit $shortsha1 && ! is_merge_commit $shortsha1
 	then
 		comment_out="# "
 	else