Fix "arc:this" and "arc:amended" base rules after D4838

Summary:
After D4383, we escape the base commit when constructing a command like this:

  hg log --rev (base::. - base)

However, if the base commit is a revset like ".^", we now escape it and Mercurial looks for a commit named ".^" (a valid mercurial branch name) instead.

Fix this by returning nodes for these rules instead of revsets. The "arc:this" rule is automatically used in some operations, like "arc amend", so users can hit this during normal workflows, not just with weird `--base` rules.

Test Plan: Ran "arc amend" in a Mercurial repository, didn't fatal out.

Reviewers: DurhamGoode, sid0

Reviewed By: DurhamGoode

CC: tido, aran

Differential Revision: https://secure.phabricator.com/D4949