GIT_PUSH_CERT* env vars and update/post-update hooks...

Hi git core devs,

Any chance I could persuade you to set the GIT_PUSH_CERT* environment
variables for the update (and post-update) hooks also?

Background: gitolite "takes over" the update hook [1] for authorisation
and enforcement, and I want to avoid taking over the pre-receive hook
also in order to do this check.

The post-update is not so important; gitolite doesn't use it anyway, so
if I have to take over one of them, I may as well take over
post-receive.  I just added that for consistency.

thanks
sitaram

[1]: because it's nice to *selectively* reject refs when more than one
ref is pushed at the same time; pre-receive is "all or none".