worktree add: reject separator-only path

worktree_basename() extracts an empty basename from a path consisting
only of zero or more path separators.  We can't use that as a worktree
name.  Properly report such a path as invalid instead of triggering a
BUG that asks the user what just happened.

Original-patch-by: Matthias Aßhauer <mha1993@live.de>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/worktree.c

diff --git a/builtin/worktree.c b/builtin/worktree.c
index 1d827c4..214de50 100644
--- a/builtin/worktree.c
+++ b/builtin/worktree.c
@@ -491,6 +491,8 @@ static int add_worktree(const char *path, const char *refname,
 
 	name = worktree_basename(path, &len);
 	strbuf_add(&sb, name, path + len - name);
+	if (!sb.len)
+		die(_("invalid path '%s'"), path);
 	sanitize_refname_component(sb.buf, &sb_name);
 	if (!sb_name.len)
 		BUG("How come '%s' becomes empty after sanitization?", sb.buf);

t/t2400-worktree-add.sh

diff --git a/t/t2400-worktree-add.sh b/t/t2400-worktree-add.sh
index 58b4445..4ffdd56 100755
--- a/t/t2400-worktree-add.sh
+++ b/t/t2400-worktree-add.sh
@@ -46,6 +46,10 @@
 	test_path_is_missing .git/worktrees/zere
 '
 
+test_expect_success '"add" rejects an empty path' '
+	test_must_fail git worktree add "" HEAD
+'
+
 test_expect_success 'checking out paths not complaining about linked checkouts' '
 	(
 	cd existing_empty &&