Fix some arc/mercurial issues

Summary:

• In "arc which", we recommend "--rev x --rev ." to show changes. This is not accurate if there are uncommitted changes in the working copy. Just "--rev x" shows the correct changes (implicitly, the other end of the range is the working copy state).
• When you diff only working copy changes, we currently incorrectly identify all your open revisions as belonging to the working copy. Instead, correctly identify none of them as belonging to the working copy (in theory, we could go farther than this and do path-based identification like SVN, but with --amend in hg 2.2+ this workflow should be going away in the long run).
• If you have uncommitted working copy changes, never try to amend.

Test Plan: Ran "arc which .", "arc diff ." in a working copy with dirty changes, got better results than before.

Reviewers: dschleimer, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1507

Differential Revision: https://secure.phabricator.com/D2980