Add extension point to register JGit PostReceiveHooks

Plugins may want to get notified when a pack file is received.
For example, the quota plugin may want to cache the repository
size in the file system and update it every time a new pack is
received.

Change-Id: I0212eab6e4867819ebe7ecc7fc5532c26bb82282
Merge "Add extension point to register JGit PostReceiveHooks"