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