builtin/credential: clear credential before exit

We never release memory associated with `struct credential`. Fix this
and mark the corresponding test as leak free.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/credential.c

@@ -39,5 +39,7 @@ int cmd_credential(int argc, const char **argv, const char *prefix UNUSED)
 	} else {
 		usage(usage_msg);
 	}
+
+	credential_clear(&c);
 	return 0;
 }