Stop populating or updating working copies in observed Mercurial repositories

Summary:
Ref T12961. Fixes T4416. Currently, for observed Mercurial repositories, we build a working copy with pull -u (for "update").

This should be unnecessary, and we don't do it for hosted Mercurial repositories. We also stopped doing it years ago for Git repositories. We also don't clone Mercurial repositories with a working copy.

It's possible something has slipped through the cracks here so I'll hold this until after the release cut, but I believe there are no actual technical blockers here.

Test Plan:

• Observed a public Mercurial repository on Bitbucket.
• Let it import.
• Browsed commits, branches, file content, etc., without any apparent issues.

Reviewers: chad

Reviewed By: chad

Subscribers: cspeckmim

Maniphest Tasks: T12961, T4416

Differential Revision: https://secure.phabricator.com/D18390