Enrich "diffusion.commit.search" with identity, status, and message information

Summary:
Depends on D19657. Ref T13197. See PHI841.

This enriches the results from diffusion.commit.search with information similar to the information returned by the "commits" attachment from differential.diff.search.

Also include unreachable, imported, message, audit status, and repository PHID.

Test Plan: Called diffusion.commit.search and reviewed the results, which looked sensible.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13197

Differential Revision: https://secure.phabricator.com/D19658