sequencer: comment checked-out branch properly

`git rebase --update-ref` does not insert commands for dependent/sub-
branches which are checked out.[1]  Instead it leaves a comment about
that fact.  The comment char is hardcoded (#).  In turn the comment
line gets interpreted as an invalid command when `core.commentChar`/
`core.commentString` is in use.

† 1: See 900b50c (rebase: add --update-refs option, 2022-07-19)

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: LemmingAvalanche

sequencer.c

@@ -6382,8 +6382,9 @@ static int add_decorations_to_list(const struct commit *commit,
 		/* If the branch is checked out, then leave a comment instead. */
 		if ((path = branch_checked_out(decoration->name))) {
 			item->command = TODO_COMMENT;
-			strbuf_addf(ctx->buf, "# Ref %s checked out at '%s'\n",
-				    decoration->name, path);
+			strbuf_commented_addf(ctx->buf, comment_line_str,
+					      "Ref %s checked out at '%s'\n",
+					      decoration->name, path);
 		} else {
 			struct string_list_item *sti;
 			item->command = TODO_UPDATE_REF;

t/t3400-rebase.sh

@@ -456,4 +456,23 @@ test_expect_success 'rebase when inside worktree subdirectory' '
 	)
 '
 
+test_expect_success 'git rebase --update-ref with core.commentChar and branch on worktree' '
+	test_when_finished git branch -D base topic2 &&
+	test_when_finished git checkout main &&
+	test_when_finished git branch -D wt-topic &&
+	test_when_finished git worktree remove wt-topic &&
+	git checkout main &&
+	git checkout -b base &&
+	git checkout -b topic2 &&
+	test_commit msg2 &&
+	git worktree add wt-topic &&
+	git checkout base &&
+	test_commit msg3 &&
+	git checkout topic2 &&
+	GIT_SEQUENCE_EDITOR="cat >actual" git -c core.commentChar=% \
+		 rebase -i --update-refs base &&
+	test_grep "% Ref refs/heads/wt-topic checked out at" actual &&
+	test_grep "% Ref refs/heads/topic2 checked out at" actual
+'
+
 test_done