dir.c: stop ignoring opendir() error in open_cached_dir()

A follow-up to the recently fixed bugs in the untracked
invalidation. If opendir() fails it should show a warning, perhaps
this should die, but if this ever happens the error is probably
recoverable for the user, and dying would just make things worse.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

dir.c

@@ -1787,11 +1787,16 @@ static int open_cached_dir(struct cached_dir *cdir,
 			   struct strbuf *path,
 			   int check_only)
 {
+	const char *c_path;
+
 	memset(cdir, 0, sizeof(*cdir));
 	cdir->untracked = untracked;
 	if (valid_cached_dir(dir, untracked, istate, path, check_only))
 		return 0;
-	cdir->fdir = opendir(path->len ? path->buf : ".");
+	c_path = path->len ? path->buf : ".";
+	cdir->fdir = opendir(c_path);
+	if (!cdir->fdir)
+		warning_errno(_("could not open directory '%s'"), c_path);
 	if (dir->untracked) {
 		invalidate_directory(dir->untracked, untracked);
 		dir->untracked->dir_opened++;