rebase -i: do not leave a CHERRY_PICK_HEAD file behind

When skipping commits whose changes were already applied via `git rebase
--continue`, we need to clean up said file explicitly.

The same is not true for `git rebase --skip` because that will execute
`git reset --hard` as part of the "skip" handling in git-rebase.sh, even
before git-rebase--interactive.sh is called.
Signed-off-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-rebase--interactive.sh

@@ -836,7 +836,11 @@ continue)
 	# do we have anything to commit?
 	if git diff-index --cached --quiet HEAD --
 	then
-		: Nothing to commit -- skip this
+		# Nothing to commit -- skip this commit
+
+		test ! -f "$GIT_DIR"/CHERRY_PICK_HEAD ||
+		rm "$GIT_DIR"/CHERRY_PICK_HEAD ||
+		die "Could not remove CHERRY_PICK_HEAD"
 	else
 		if ! test -f "$author_script"
 		then

t/t3404-rebase-interactive.sh

@@ -1039,7 +1039,7 @@ test_expect_success 'short SHA-1 collide' '
 	)
 '
 
-test_expect_failure 'rebase --continue removes CHERRY_PICK_HEAD' '
+test_expect_success 'rebase --continue removes CHERRY_PICK_HEAD' '
 	git checkout -b commit-to-skip &&
 	for double in X 3 1
 	do