assume parse_commit checks for NULL commit

The parse_commit function will check whether it was passed a
NULL commit pointer, and if so, return an error. There is no
need for callers to check this separately.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -79,7 +79,7 @@ struct commit *lookup_commit_reference_by_name(const char *name)
 	if (get_sha1_committish(name, sha1))
 		return NULL;
 	commit = lookup_commit_reference(sha1);
-	if (!commit || parse_commit(commit))
+	if (parse_commit(commit))
 		return NULL;
 	return commit;
 }