R1:45e93c8f1d1a

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`:

{F6020742}

Same commit is not on `stable`:

{F6020743}

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13216

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