cache: move cache invalidation from web handler to post push hook

We need the post push hook anyway ... and having the cache invalidation here
will also work for ssh pushes in the future.

The name log_push_action is thus no longer spot-on. That might change later,
but requires some care as it also is used directly as hook name.

Note that having cache invalidation in the hook will do that debug logging no
longer will appear in the server log.

Based on a patch by Dominik Ruf.