mail.sh: support hooks.reverseorder for show_new_revisions
By default new revisions are shown in Git's standard newest to oldest
order. When reading log messages that refer to changes made in older
commits this requires reading the notification email from back-to-front
in order to make sense of such things.
Provide a "hooks.reverseorder" option so that new revisions can be shown
in oldest to newest order instead in order to provide a more natural
reading experience when viewing notification emails.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>