R1:3706ec4879a1

Promote `phutil_tag`

Summary: Fixes T2432. Anything else we should do before we land this? I'll land the lint rules at the same time.

Test Plan: See D4807.

Reviewers: vrana, btrahan

CC: aran, lesha, edward

Maniphest Tasks: T2432

Differential Revision: https://secure.phabricator.com/D4822