format-patch: free elements of rev.ref_message_ids list

format-patch: free elements of rev.ref_message_ids list

When we are showing multiple patches with format-patch, we have to
repeatedly overwrite the rev.message_id field. We take care to avoid
leaking the old value by either freeing it, or adding it to
ref_message_ids, a string list of ids to reference in subsequent
messages.

But unfortunately we do leak the value via that string list. We try
to clear the string list, courtesy of 89f45cf4eb (format-patch: don't
leak "extra_headers" or "ref_message_ids", 2022-04-13). But since it was
initialized as "nodup", the string list doesn't realize it owns the
strings, and it leaks them.

We have two options here:

  1. Continue to init with "nodup", but then tweak the value of
     ref_message_ids.strdup_strings just before clearing.

  2. Init with "dup", but use "append_nodup" when transferring ownership
     of strings to the list. Clearing just works.

I picked the second here, as I think it calls attention to the tricky
part (transferring ownership via the nodup call).

There's one other related fix we have to do, though. We also insert the
result of clean_message_id() into the list. This _sometimes_ allocates
and sometimes does not, depending on whether we have to remove cruft
from the end of the string. Let's teach it to consistently return an
allocated string, so that the caller knows it must be freed.

There's no new test here, as the leak can already be seen in t4014.44 (as
well as others in that script). We can't mark all of t4014 as leak-free,
though, as there are other unrelated leaks that it triggers.

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