Attach identities to commits and users to identities

Summary: Ref T12164. Make it easier to work with identity objects by attaching them to commits and attaching users to identities.

Test Plan: Loaded some commits with ->needIdentities(true) and checked the resulting objects.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12164

Differential Revision: https://secure.phabricator.com/D19491