branch: strip refs/heads/ from the --set-upstream-to hint

When git switch fails to write the tracking config, it prints a hint
with the command to set the upstream by hand. For example, if
.git/config.lock exists:

    error: could not lock config file .git/config: File exists
    error: unable to write upstream branch configuration
    hint:
    hint: After fixing the error cause you may try to fix up
    hint: the remote tracking information by invoking:
    hint:   git branch --set-upstream-to=origin/refs/heads/feat

The hint says origin/refs/heads/feat, but the correct name is
origin/feat. Running the command as printed fails, because the full
ref name is not a valid remote-tracking branch name here.

This has been wrong since a3f40ec4b0 (branch: accept multiple upstream
branches for tracking, 2021-12-20).

Strip refs/heads/ from the hint, as the success message in the same
function already does. The git config --add line for multiple upstreams
still needs the full ref name, so it is not changed.

Signed-off-by: tsukasa inoue <inoue.tsukasa.4@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

branch.c

@@ -186,12 +186,14 @@ out_err:
 
 	advise(_("\nAfter fixing the error cause you may try to fix up\n"
 		"the remote tracking information by invoking:"));
-	if (remotes->nr == 1)
+	if (remotes->nr == 1) {
+		shortname = remotes->items[0].string;
+		skip_prefix(shortname, "refs/heads/", &shortname);
 		advise("  git branch --set-upstream-to=%s%s%s",
 			origin ? origin : "",
 			origin ? "/" : "",
-			remotes->items[0].string);
-	else {
+			shortname);
+	} else {
 		advise("  git config --add branch.\"%s\".remote %s",
 			local, origin ? origin : ".");
 		for_each_string_list_item(item, remotes)

t/t3200-branch.sh

@@ -1080,6 +1080,31 @@ test_expect_success '--set-upstream-to fails on locked config' '
 	test_grep "could not lock config file .git/config" err
 '
 
+test_expect_success '--set-upstream-to hint uses short remote-tracking branch name on locked config' '
+	test_when_finished "git branch -D hint-local" &&
+	test_when_finished "git update-ref -d refs/remotes/hintremote/hintbranch" &&
+	test_config remote.hintremote.url . &&
+	test_config remote.hintremote.fetch "+refs/heads/*:refs/remotes/hintremote/*" &&
+	git update-ref refs/remotes/hintremote/hintbranch HEAD &&
+	git branch hint-local &&
+	test_when_finished "rm -f .git/config.lock" &&
+	>.git/config.lock &&
+	test_must_fail git -c core.configLockTimeout=0 \
+		branch --set-upstream-to=hintremote/hintbranch hint-local 2>err &&
+	cat >expect <<-\EOF &&
+	hint:
+	hint: After fixing the error cause you may try to fix up
+	hint: the remote tracking information by invoking:
+	hint:   git branch --set-upstream-to=hintremote/hintbranch
+	EOF
+	grep "^hint:" err >actual &&
+	test_cmp expect actual &&
+	rm -f .git/config.lock &&
+	git branch --set-upstream-to=hintremote/hintbranch hint-local &&
+	test_cmp_config hintremote branch.hint-local.remote &&
+	test_cmp_config refs/heads/hintbranch branch.hint-local.merge
+'
+
 test_expect_success 'use --set-upstream-to modify HEAD' '
 	test_config branch.main.remote foo &&
 	test_config branch.main.merge foo &&