commit: allow prepare_commit_graft to handle arbitrary repositories

Move the global variable 'commit_graft_prepared' into the object
pool and convert the function prepare_commit_graft to work
an arbitrary repositories.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

object.h

@@ -20,6 +20,8 @@ struct parsed_object_pool {
 	int is_shallow;
 	struct stat_validity *shallow_stat;
 	char *alternate_shallow_file;
+
+	int commit_graft_prepared;
 };
 
 struct parsed_object_pool *parsed_object_pool_new(void);