Merge branch 'jc/send-email-pre-process-fix'
When "git send-email" that uses the validate hook is fed a message
without and then with Message-ID, it failed to auto-assign a unique
Message-ID to the former and instead reused the Message-ID from the
latter, which has been corrected. This was a fix for a recent
regression caught before the release, so no need to mention it in
the release notes.
* jc/send-email-pre-process-fix:
t9001: mark the script as no longer leak checker clean
send-email: clear the $message_id after validation