worktree: allow the main brach of a bare repository to be checked out

In bare repositories, get_worktrees() still returns the main repository,
so git worktree list can show it. ignore it in find_shared_symref so we
can still check out the main branch.

Signed-off-by: Dennis Kaarsemaker <dennis@kaarsemaker.net>
Acked-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t2025-worktree-add.sh

@@ -138,6 +138,14 @@ test_expect_success 'checkout from a bare repo without "add"' '
 	)
 '
 
+test_expect_success '"add" default branch of a bare repo' '
+	(
+		git clone --bare . bare2 &&
+		cd bare2 &&
+		git worktree add ../there3 master
+	)
+'
+
 test_expect_success 'checkout with grafts' '
 	test_when_finished rm .git/info/grafts &&
 	test_commit abc &&

worktree.c

@@ -345,6 +345,8 @@ const struct worktree *find_shared_symref(const char *symref,
 
 	for (i = 0; worktrees[i]; i++) {
 		struct worktree *wt = worktrees[i];
+		if (wt->is_bare)
+			continue;
 
 		if (wt->is_detached && !strcmp(symref, "HEAD")) {
 			if (is_worktree_being_rebased(wt, target)) {