stash: strip "refs/heads/" with skip_prefix

When generating a message for a stash, "git stash" only records the
part of the branch name to the right of the last "/". e.g. if HEAD is at
"foo/bar/baz", "git stash" generates a message prefixed with "WIP on
baz:" instead of "WIP on foo/bar/baz:".

Fix this by using skip_prefix() to skip "refs/heads/" instead of looking
for the last instance of "/".

Reported-by: Kraymer <kraymer@gmail.com>
Reported-by: Daniel Hahler <git@thequod.de>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/stash.c

@@ -1327,7 +1327,7 @@ static int do_create_stash(const struct pathspec *ps, struct strbuf *stash_msg_b
 
 	branch_ref = resolve_ref_unsafe("HEAD", 0, NULL, &flags);
 	if (flags & REF_ISSYMREF)
-		branch_name = strrchr(branch_ref, '/') + 1;
+		skip_prefix(branch_ref, "refs/heads/", &branch_name);
 	head_short_sha1 = find_unique_abbrev(&head_commit->object.oid,
 					     DEFAULT_ABBREV);
 	strbuf_addf(&msg, "%s: %s ", branch_name, head_short_sha1);

t/t3903-stash.sh

@@ -1042,6 +1042,17 @@ test_expect_success 'create stores correct message' '
 	test_cmp expect actual
 '
 
+test_expect_success 'create when branch name has /' '
+	test_when_finished "git checkout main" &&
+	git checkout -b some/topic &&
+	>foo &&
+	git add foo &&
+	STASH_ID=$(git stash create "create test message") &&
+	echo "On some/topic: create test message" >expect &&
+	git show --pretty=%s -s ${STASH_ID} >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'create with multiple arguments for the message' '
 	>foo &&
 	git add foo &&