format-patch: don't leak "extra_headers" or "ref_message_ids"

format-patch: don't leak "extra_headers" or "ref_message_ids"

Fix two memory leaks in "struct rev_info" by freeing that memory in
cmd_format_patch(). These two are unusual special-cases in being in
the "struct rev_info", but not being "owned" by the code in
revision.c. I.e. they're members of the struct so that this code in
"builtin/log.c" can conveniently pass information code in
"log-tree.c".

See e.g. the make_cover_letter() caller of log_write_email_headers()
here in "builtin/log.c", and [1] for a demonstration of where the
"extra_headers" and "ref_message_ids" struct members are used.

See 20ff06805c6 (format-patch: resurrect extra headers from config,
2006-06-02) and d1566f7883f (git-format-patch: Make the second and
subsequent mails replies to the first, 2006-07-14) for the initial
introduction of "extra_headers" and "ref_message_ids".

We can count on repo_init_revisions() memset()-ing this data to 0
however, so we can count on it being either NULL or something we
allocated. In the case of "extra_headers" let's add a local "char *"
variable to hold it, to avoid the eventual cast from "const char *"
when we free() it.

1. https://lore.kernel.org/git/220401.868rsoogxf.gmgdl@evledraar.gmail.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>