fast-export: don't parse commits while reading marks file

We don't need the parsed objects at this point, merely the
information that they have marks.

Seems to be three times faster in my setup with lots of objects.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fast-export.c

@@ -613,6 +613,7 @@ static void import_marks(char *input_file)
 		char *line_end, *mark_end;
 		unsigned char sha1[20];
 		struct object *object;
+		struct commit *commit;
 		enum object_type type;
 
 		line_end = strchr(line, '\n');
@@ -636,7 +637,11 @@ static void import_marks(char *input_file)
 			/* only commits */
 			continue;
 
-		object = parse_object(sha1);
+		commit = lookup_commit(sha1);
+		if (!commit)
+			die("not a commit? can't happen: %s", sha1_to_hex(sha1));
+
+		object = &commit->object;
 
 		if (object->flags & SHOWN)
 			error("Object %s already has a mark", sha1_to_hex(sha1));