fsck: ignore missing "refs" directory for linked worktrees

"git refs verify" doesn't work if there are worktrees created on Git
v2.43.0 or older versions. These versions don't automatically create the
"refs" directory, causing the error:

    error: cannot open directory .git/worktrees/<worktree name>/refs:
    No such file or directory

Since 8f4c00d (builtin/worktree: create refdb via ref backend,
2024-01-08), we automatically create the "refs" directory for new
worktrees. And in 7c78d81 (ref: support multiple worktrees check for
refs, 2024-11-20), we assume that all linked worktrees have this
directory and would wrongly report an error to the user, thus
introducing compatibility issue.

Check for ENOENT errno before reporting directory access errors for
linked worktrees to maintain backward compatibility.

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

Author: shejialuo

refs/files-backend.c

@@ -3772,6 +3772,9 @@ static int files_fsck_refs_dir(struct ref_store *ref_store,
 
 	iter = dir_iterator_begin(sb.buf, 0);
 	if (!iter) {
+		if (errno == ENOENT && !is_main_worktree(wt))
+			goto out;
+
 		ret = error_errno(_("cannot open directory %s"), sb.buf);
 		goto out;
 	}

t/t0602-reffiles-fsck.sh

@@ -106,6 +106,25 @@ test_expect_success 'ref name check should be adapted into fsck messages' '
 	test_must_be_empty err
 '
 
+test_expect_success 'no refs directory of worktree should not cause problems' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit initial &&
+		git worktree add --detach ./worktree &&
+
+		(
+			cd worktree &&
+			worktree_refdir="$(git rev-parse --git-dir)/refs" &&
+			# Simulate old directory layout
+			rmdir "$worktree_refdir" &&
+			git refs verify 2>err &&
+			test_must_be_empty err
+		)
+	)
+'
+
 test_expect_success 'ref name check should work for multiple worktrees' '
 	test_when_finished "rm -rf repo" &&
 	git init repo &&