update-hook: Major overhaul (handling tags, mainly).

update-hook: Major overhaul (handling tags, mainly).

This is the update hook we use in all our git-repos.

It has some improvements over the original version, namely:
* Don't send every commit since dawn of time when adding a new tag.
* When updating an annotated tag, just send the diffs since the last tag.
* Add diffstat output for 'normal' commits (top) and annotated tags (bottom).
* Block un-annotated tags in shared repos.

I'm a bit uncertain about that last one, but it demonstrates how to
disallow updates of a ref which we use, so I kept it.

Note that git-describe is needed for the "changes since last annotated tag"
thing to work.

Signed-off-by: Andreas Ericsson <ae@op5.se>
Signed-off-by: Junio C Hamano <junkio@cox.net>