Assign PHIDs to PushLogs

Summary: Ref T4195. We need these in Herald, since HeraldTranscripts need to refer to a PHID which they acted upon.

Test Plan:
Ran migration, got PHIDs:

  mysql> select phid from repository_pushlog limit 3;
  +--------------------------------+
  | phid                           |
  +--------------------------------+
  | PHID-PSHL-25jnc6cjgzw5rwqgmr7r |
  | PHID-PSHL-2vrvmtslkrj5yv7nxsv2 |
  | PHID-PSHL-34x262zkrwoka6mplony |
  +--------------------------------+
  3 rows in set (0.00 sec)

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4195

Differential Revision: https://secure.phabricator.com/D7780