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>

builtin-pack-objects.c

@@ -673,7 +673,7 @@ static void setup_delta_attr_check(struct git_attr_check *check)
 	static struct git_attr *attr_delta;
 
 	if (!attr_delta)
-		attr_delta = git_attr("delta", 5);
+		attr_delta = git_attr("delta");
 
 	check[0].attr = attr_delta;
 }