Clean up hg --debug branches calls

Summary: Ref T1493. Consolidate these a bit; they might need some more magic once we do --noupdate checkouts. Mostly just trying to clean up and centralize this code a bit.

Test Plan: Viewed and bin/repository discover'd Mercurial repos with and without any branches.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1493

Differential Revision: https://secure.phabricator.com/D7480