Summary:
Pull --rebase when you have local draft commits moves your commits,
your working copy, and your active bookmark to the new branch head
(or whatever destination you specify with -d). If you don't have
any local draft commits, it doesn't do any of that.
This change makes it so that it does everything except moving the
commits.
Test Plan: Added new unit tests. Re-ran existing ones.
Reviewers: #fbhgext, ryanmce
Differential Revision: https://phab.mercurial-scm.org/D518
