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>
diff --git a/archive.c b/archive.c
index 55b2732..a9ebdc5 100644
--- a/archive.c
+++ b/archive.c
@@ -87,8 +87,8 @@
 	static struct git_attr *attr_export_subst;
 
 	if (!attr_export_ignore) {
-		attr_export_ignore = git_attr("export-ignore", 13);
-		attr_export_subst = git_attr("export-subst", 12);
+		attr_export_ignore = git_attr("export-ignore");
+		attr_export_subst = git_attr("export-subst");
 	}
 	check[0].attr = attr_export_ignore;
 	check[1].attr = attr_export_subst;