Documentation: shared repository management in tutorial.

Documentation: shared repository management in tutorial.

The branch policy script I outlined was improved and polished by
Carl and posted on the list twice since then.  It is a shame not
to pick it up, so replace the original outline in
howto/update-hook-example.txt with the latest from Carl.

Also talk about setting up git-shell to allow git-push/git-fetch
only SSH access to a shared repository host in the tutorial.

Signed-off-by: Junio C Hamano <junkio@cox.net>