format-patch: use GIT_COMMITTER_EMAIL in message ids

Before commit 43ae9f47, we generated the tail of a message id
by calling git_committer_info and parsing the email out of
the result. 43ae9f47 changed to use ident_default_email
directly, so we didn't have to bother with parsing. As a
side effect, it meant we no longer used GIT_COMMITTER_EMAIL
at all.

In general, this is probably reasonable behavior. Either the
default email is sane on your system, or you are using
user.email to provide something sane. The exception is if
you rely on GIT_COMMITTER_EMAIL being set all the time to
override the bogus generated email.

This is unlikely to match anybody's real-life setup, but we
do use it in the test environment. And furthermore, it's
what we have always done, and the change in 43ae9f47 was
about cleaning up, not fixing any bug; we should be
conservative and keep the behavior identical.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/log.c

@@ -740,7 +740,8 @@ static void gen_message_id(struct rev_info *info, char *base)
 {
 	struct strbuf buf = STRBUF_INIT;
 	strbuf_addf(&buf, "%s.%lu.git.%s", base,
-		    (unsigned long) time(NULL), ident_default_email());
+		    (unsigned long) time(NULL),
+		    git_committer_info(IDENT_NO_NAME|IDENT_NO_DATE));
 	info->message_id = strbuf_detach(&buf, NULL);
 }