Heavily expanded update hook to send more useful emails than the old hook

Heavily expanded update hook to send more useful emails than the old hook

I know it's only an example, but having this might save someone else the
trouble of writing an enhanced version for themselves.

It basically does the same job as the old update hook, but with these
differences:
 * The recipients list is read from the repository config file from
   hooks.mailinglist
 * Updating unannotated tags can be allowed by setting
   hooks.allowunannotated
 * Announcement emails (via annotated tag creation) can be sent to a
   different mailing list by setting hooks.announcelist
 * Output email is more verbose and generates specific content depending
   on whether the ref is a tag, an annotated tag, a branch, or a
   tracking branch
 * The email is easier to filter; the subject line is prefixed with
   [SCM] and a project description pulled from the "description" file
 * It catches (and displays differently) branch updates that are
   performed with a --force

Obviously, it's nothing that clever - it's the update hook I use on my
repositories but I've tried to keep it general, and tried to make the
output always relevant to the type of update.

Signed-off-by: Andy Parkins <andyparkins@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>