R1:a5f55d506f46

Provide a real object ("PhabricatorRepositoryPushEvent") to represent an entire push transaction

Summary:
Ref T4677. Currently, we record individual actions in a push as PhabricatorRepositoryPushLogs, but tie them together only loosely with a `transactionKey`.

Provide a real PushEvent object, and move some of the denormalized fields to it. This primarily just gives us more robust infrastructure for building, e.g., email about pushes, for T4677, since we can act on real PHIDs rather than passing awkward…