Use repository identities, not denormalized strings, to identify authors for "Revision closed by commit X" stories

Summary:
Ref T12164. Ref T13276. Currently, the parsing pipeline copies the author and committer names and PHIDs into the transcaction record as metadata. They are then rendered directly from the metadata.

This makes planned changes to the parsing pipeline (to prevent races when multiple commits matching a single revision are pushed simultaneously) more difficult, and generally won't work with repository identities.

Instead, load the commit and use its identities.

Test Plan: Loaded a revision, saw the same story rendering for a "Closed by commit" story.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13276, T12164

Differential Revision: https://secure.phabricator.com/D20418