RFE: execute post-receive hook on fetch

Hi,

It would be nice, if git could execute the "post-receive" hook (or a hook wit a new name but similar functionality) when doing a fetch.

This would be really useful in setups where e.g. a buildbot has its own (bare) mirror of a project's repository, polls on the main repo and now needs to know what was fetched. Currently it has to record old refs and compare them with the refs itself. It could also inspect FETCH_HEADS to see what was fetched, but still, this file doesn't record old values for each ref.

The post-receive hook is executed when pushing stuff to the buildbot's mirror, but not when it fetches new stuff itself. This is kind of asymmetrical, and I didn't expect that no hook is executed in that case.

- Thomas