format-patch: return an allocated string from log_write_email_headers()

format-patch: return an allocated string from log_write_email_headers()

When pretty-printing a commit in the email format, we have to fill in
the "after subject" field of the pretty_print_context with any extra
headers the user provided (e.g., from "--to" or "--cc" options) plus any
special MIME headers.

We return an out-pointer that sometimes points to a newly heap-allocated
string and sometimes not. To avoid leaking, we store the allocated
version in a buffer with static lifetime, which is ugly. Worse, as we
extend the header feature, we'll end up having to repeat this ugly
pattern.

Instead, let's have our out-pointer pass ownership back to the caller,
and duplicate the string when necessary. This does mean one extra
allocation per commit when you use extra headers, but in the context of
format-patch which is showing diffs, I don't think that's even
measurable.

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