(stable) Change "Revision Close" story to use commit identities only with no fallback to commit data

Summary:
See PHI1213. If we don't have identities for "revision X closed by commit Y" stories, just do the plain non-attribution rendering rather than trying to fall back. Falling back won't work since we don't load the data, which should be OK now since identities seem like they're in generally good shape.

(We could probably just throw out the fallback behavior instead, but we can always clean things up later.)

Test Plan: Forced no commit identity on a revision, loaded it, saw reasonable story rendering.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20460