patch-ids: add flag to create the diff patch id using header only data

This will allow a diff patch id to be created using only the header data
so that the contents of the file will not have to be loaded.

Signed-off-by: Kevin Willford <kcwillford@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/patch-ids.c b/patch-ids.c
index bafaae2..69a14a3 100644
--- a/patch-ids.c
+++ b/patch-ids.c
@@ -13,7 +13,7 @@
 	else
 		diff_root_tree_sha1(commit->object.oid.hash, "", options);
 	diffcore_std(options);
-	return diff_flush_patch_id(options, sha1);
+	return diff_flush_patch_id(options, sha1, 0);
 }
 
 static int patch_id_cmp(struct patch_id *a,