Add a "tags" field to Diffusion commit

Summary:
  - Fixes T4588.
  - See D8501.
  - Adds a "Tags" field for Herald commit emails.
  - Fixes a bug in `tagsquery` when filtering by commit name.
  - Make `tagsquery` just return nothing instead of fataling against Mercurial/Subversion.

Test Plan: Used `bin/repository/reparse.php --herald` to exercise this code.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: aran, epriestley

Maniphest Tasks: T4588

Differential Revision: https://secure.phabricator.com/D8502