setup: return correct prefix if worktree is '/'

The same old problem reappears after setup code is reworked.  We tend
to assume there is at least one path component in a path and forget
that path can be simply '/'.

Reported-by: Matthijs Kooijman <matthijs@stdin.nl>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/dir.h b/dir.h
index 72a764ed84..86a99ed9c1 100644
--- a/dir.h
+++ b/dir.h
@@ -83,6 +83,7 @@ extern int file_exists(const char *);
 
 extern char *get_relative_cwd(char *buffer, int size, const char *dir);
 extern int is_inside_dir(const char *dir);
+extern int dir_inside_of(const char *subdir, const char *dir);
 
 static inline int is_dot_or_dotdot(const char *name)
 {