Add extension point for receive-pack initialization.

Plugins can register ReceivePackInitializer's in order to provide custom
initialization of the ReceivePack instance. For example, the quota plugin
may use this extension point to set the maximum allowed pack size based
on its quota configuration and the disk space occupied by that project.

Change-Id: I8d6e7d4bb75099b0fa5f6968ae5371883bf865ee
5 files changed