Don't track "phabricator/" staging area tags

Summary: Ref T9028. Ref T6878. This rule should probably be refined in the long term, but for now just ignore "phabricator/diff/12424" and similar staging area tags.

Test Plan: Ran bin/repository discover --verbose on a repository with staging area refs, saw Phabricator ignore those refs as untracked.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T6878, T9028

Differential Revision: https://secure.phabricator.com/D16134