Contribute a fairly paranoid update hook

Contribute a fairly paranoid update hook

I'm using a variant of this update hook in a corporate environment
where we perform some validations of the commits and tags that
are being pushed.  The model is a "central repository" type setup,
where users are given access to push to specific branches within
the shared central repository.  In this particular installation we
run a specially patched git-receive-pack in setuid mode via SSH,
allowing all writes into the repository as the repository owner,
but only if this hook blesses it.

One of the major checks we perform with this hook is that the
'committer' line of a commit, or the 'tagger' line of a new annotated
tag actually correlates to the UNIX user who is performing the push.
Users can falsify these lines on their local repositories, but
the central repository that management trusts will reject all such
forgery attempts.  Of course 'author' lines are still allowed to
be any value, as sometimes changes do come from other individuals.

Another nice feature of this hook is the access control lists for
all repositories on the system can also be stored and tracked in
a supporting Git repository, which can also be access controlled
by itself.  This allows full auditing of who-had-what-when-and-why,
thanks to git-blame's data mining capabilities.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>