git_attr(): fix function signature

The function took (name, namelen) as its arguments, but all the public
callers wanted to pass a full string.

Demote the counted-string interface to an internal API status, and allow
public callers to just pass the string to the function.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

userdiff.c

@@ -198,7 +198,7 @@ struct userdiff_driver *userdiff_find_by_path(const char *path)
 	struct git_attr_check check;
 
 	if (!attr)
-		attr = git_attr("diff", 4);
+		attr = git_attr("diff");
 	check.attr = attr;
 
 	if (!path)