Expose "identifiers" as a query constraint for Commit search

Summary: Ref T13216. See PHI984. The CommitSearchEngine (and, by extension, diffusion.commit.search) currently do not support identifier search, but this is a reasonable capability to provide.

Test Plan:
Testing that a commit exists on master:

Same commit is not on stable:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13216

Differential Revision: https://secure.phabricator.com/D19825