fbamend: add hide - a simple command for hiding commits

Summary:
We have two commands for hiding commits:

• strip - which has a lot of legacy options and legacy name
• prune - which allows for creating arbitrary obsolescence markers

Both of them have complicated UI and are suboptimal experiences for unadvanced
user. The hg hide command is very simple, does one job and does it well.

Test Plan: see test

Reviewers: #fbhgext, durham, quark, kulshrax

Reviewed By: #fbhgext, quark

Subscribers: akushner

Differential Revision: https://phab.mercurial-scm.org/D178