Summary: Added a configuration option to make fbamend use the functionality from `hg rebase --restack` instead of the previous bookmark-based approach in `hg amend`.
Test Plan: Enable the `fbamend.userestack` config option and attempt to use `hg amend`, `hg amend --fixup`, and `hg amend --rebase`. These should all work just as before, except that `hg amend` will no longer create .preamend bookmarks.
Reviewers: #sourcecontrol, quark, durham
Reviewed By: durham
Subscribers: mjpieters
Differential Revision: https://phabricator.intern.facebook.com/D4062925
Tasks: 13839177
Signature: t1:4062925:1477359251:0ece10599987d6b8e39384e9530866ffb2e50e58
