R1:2c150076b00f

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…