attr: warn on inaccessible attribute files

Just like config and gitignore files, we silently ignore
missing or inaccessible attribute files. An existent but
inaccessible file is probably a configuration error, so
let's warn the user.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

attr.c

@@ -352,8 +352,11 @@ static struct attr_stack *read_attr_from_file(const char *path, int macro_ok)
 	char buf[2048];
 	int lineno = 0;
 
-	if (!fp)
+	if (!fp) {
+		if (errno != ENOENT)
+			warning(_("unable to access '%s': %s"), path, strerror(errno));
 		return NULL;
+	}
 	res = xcalloc(1, sizeof(*res));
 	while (fgets(buf, sizeof(buf), fp))
 		handle_attr_line(res, buf, path, ++lineno, macro_ok);