rebase -i -p: leave a --cc patch when a merge could not be redone

The result is easier to review this way, and the merge resolution has to be
done inside the work tree, not by adjusting "the patch" anyway.

git-rebase--interactive.sh

diff --git a/git-rebase--interactive.sh b/git-rebase--interactive.sh
index 89c39eb..c8b0861 100755
--- a/git-rebase--interactive.sh
+++ b/git-rebase--interactive.sh
@@ -115,9 +115,18 @@
 }
 
 make_patch () {
-	parent_sha1=$(git rev-parse --verify "$1"^) ||
-		die "Cannot get patch for $1^"
-	git diff-tree -p "$parent_sha1".."$1" > "$DOTEST"/patch
+	sha1_and_parents="$(git rev-list --parents -1 "$1")"
+	case "$sha1_and_parents" in
+	?*' '?*' '?*)
+		git diff --cc $sha1_and_parents
+		;;
+	?*' '?*)
+		git diff-tree -p "$1^!"
+		;;
+	*)
+		echo "Root commit"
+		;;
+	esac > "$DOTEST"/patch
 	test -f "$DOTEST"/message ||
 		git cat-file commit "$1" | sed "1,/^$/d" > "$DOTEST"/message
 	test -f "$DOTEST"/author-script ||

t/t3409-rebase-preserve-merges.sh

diff --git a/t/t3409-rebase-preserve-merges.sh b/t/t3409-rebase-preserve-merges.sh
index 820e010..e6c8327 100755
--- a/t/t3409-rebase-preserve-merges.sh
+++ b/t/t3409-rebase-preserve-merges.sh
@@ -83,6 +83,7 @@
 	test 2 = $(git ls-files B | wc -l) &&
 	echo Resolved again > B &&
 	test_must_fail git rebase --continue &&
+	grep "^@@@ " .git/rebase-merge/patch &&
 	git add B &&
 	git rebase --continue &&
 	test 1 = $(git rev-list --all --pretty=oneline | grep "Modify A" | wc -l) &&