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