[PATCH] git-commit --amend: respect grafted parents.

This allows a poor-mans-filter-branch of the top-most commit.

Specifically, I had this history:

   --o--o--A'    <-- amended version of A
         \
          A
           \
    --o--B--M

I now wanted to "amend" M to pull in A' instead of A as its second parent.
For various reasons I didn't want to redo the merge M again, in particular,
it already contained the changes that were amended into A'. So I figured
I would just install a graft that lists B and A' as parents of M and then
do a simple git commit --amend. Alas, git commit looks at the real parents
instead of the grafted ones, so the amended M' would still have A as its
second parent. Here is the fix that picks the grafted parents instead.

Signed-off-by: Johannes Sixt <redacted>
---
  I know that git-commit is becoming a builtin. At the least, here is
  a test that makes sure that the behavior is remains.

  -- Hannes

 git-commit.sh     |    4 ++--
 t/t7501-commit.sh |    9 +++++++++
 2 files changed, 11 insertions(+), 2 deletions(-)
 mode change 100644 => 100755 t/t7501-commit.sh

diff --git a/git-commit.sh b/git-commit.sh
index 7a7a2cb..33e7503 100755
--- a/git-commit.sh
+++ b/git-commit.sh

@@ -529,8 +529,8 @@ then
 		PARENTS="-p HEAD "`sed -e 's/^/-p /' "$GIT_DIR/MERGE_HEAD"`
 	elif test -n "$amend"; then
 		rloga='commit (amend)'
-		PARENTS=$(git cat-file commit HEAD |
-			sed -n -e '/^$/q' -e 's/^parent /-p /p')
+		PARENTS=$(git rev-list --parents -1 HEAD |
+			sed -e 's/^[^ ]*//' -e 's/ / -p /g')
 	fi
 	current="$(git rev-parse --verify HEAD)"
 else

diff --git a/t/t7501-commit.sh b/t/t7501-commit.sh
old mode 100644
new mode 100755
index b151b51..09c0d98
--- a/t/t7501-commit.sh
+++ b/t/t7501-commit.sh

@@ -163,4 +163,13 @@ test_expect_success 'partial commit that involves removal (3)' '
 
 '
 
+test_expect_success '--amend respects grafts' '
+
+	grandparent=$(git rev-parse HEAD~2) &&
+	echo $(git rev-parse HEAD) $grandparent > .git/info/grafts &&
+	git commit --amend -C HEAD &&
+	test "parent $grandparent" = "$(git cat-file commit HEAD | grep "^parent")"
+
+'
+
 test_done

-- 
1.5.3.3.gcc9e