http-push: clear refspecs before exiting

We parse the command-line arguments into a refspec struct, but we never
free them. We should do so before exiting to avoid triggering the
leak-checker.

This triggers in t5540 many times (basically every invocation of
http-push).

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

http-push.c

@@ -1983,5 +1983,7 @@ int cmd_main(int argc, const char **argv)
 		request = next_request;
 	}
 
+	refspec_clear(&rs);
+
 	return rc;
 }