Give Mercurial more plausible marker behavior

Summary: Ref T13546. Fixes some issues where marker selection in Mercurial didn't work, and selects "draft()" as the set of commits to show, which is at least somewhat reasonable.

Test Plan: Ran "arc branches" and "arc bookmarks" in Mercurial, got more reasonable output.

Maniphest Tasks: T13546

Differential Revision: https://secure.phabricator.com/D21380