format-patch: free rev.message_id when exiting

format-patch: free rev.message_id when exiting

We may allocate a message-id string via gen_message_id(), but we never
free it, causing a small leak. This can be demonstrated by running t9001
with a leak-checking build. The offending test is the one touched by
3ece9bf0f9 (send-email: clear the $message_id after validation,
2023-05-17), but the leak is much older than that. The test was simply
unlucky enough to trigger the leaking code path for the first time.

We can fix this by freeing the string at the end of the function. We can
also re-mark the test script as leak-free, effectively reverting
20bd08aefb (t9001: mark the script as no longer leak checker clean,
2023-05-17).

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