R1:925a0d3d5918

Pass repository PHID to custom hooks in PHABRICATOR_REPOSITORY instead of callsign

Summary:
Ref T4245. We pass this exclusively for use by additional third-party hooks.

This is technically a backward compatibility break, but I suspect it doesn't affect anyone:

- Probably almost no one is using this (there are few reasons to, even for the tiny number of installs with custom commit hooks).
- If they are, there's a good chance the PHID will work anyway, since nearly all scripts and Conduit methods will…