fbamend: add unhide command for easy unhiding of commits

Summary:
This command helps the user to bring back the commits to life with an explicit
command - wihtout the need to change working copy parent or create bookmarks.

Test Plan: see test

Reviewers: #fbhgext, durham, kulshrax, quark

Reviewed By: #fbhgext, quark

Subscribers: ryanmce

Differential Revision: https://phab.mercurial-scm.org/D179