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

@@ -115,9 +115,18 @@ mark_action_done () {
 }
 
 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

@@ -83,6 +83,7 @@ test_expect_success '--continue works after a conflict' '
 	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) &&