Adding arc revert command[]

Summary:
Arc Revert does the following:
1. Git revert
2. Go to the differential of the rev you are reverting and either repoen it or set it to a reverted state
3. File a hipri task to orig author

[Preview] Creating Arc Revert workflow

Porting arc revert from FB4A to phabricator for general usage. This is my first stab,
so totally appreciate feedback and assistance. I'm currently focused
on making this work for git. However, I built out the functions through the GitAPI so this
could be easily extendable to Mercurial later on.

Stuck on the following (help):
1. Creating a task for FB internal. I tried building on top of existing arc listeners
but getting errors on failures to load the TaskCreator (and other) tasks.

2. I'm using a hacky way to grab the diff revision id from the newly created
revert diff. (see line 204) I'm looking for a way to just fetch the diff ID
from arc after the diff is created; is this possible?

Test Plan:
 -

1. Ran arc revert on a www and fbcode diff
2. Confirmed that revert was run on the diffs and a proper diff filed
 -

Reviewers: royw, sdwilsh, nh, epriestley

Reviewed By: epriestley

CC: aran, epriestley, Korvin, pti, keir

Maniphest Tasks: T1751

Differential Revision: https://secure.phabricator.com/D5553