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
Remove "--allow-empty-message"

Summary: @alex has git 1.7.0.4 which doesn't have this flag. We don't actually need it: we always provide a commit message when calling this method. Remove the flag for compatibility, leaving a note in case we bump into this in the future.

Test Plan: N/A

Reviewers: btrahan, vrana, alex

Reviewed By: alex

CC: aran

Differential Revision: https://secure.phabricator.com/D5926
Arcanist changes related to T2784

Summary: These were in my sandbox, but I forgot about them. Without this things break post D5896. Ref T2784

Test Plan: my sandbox works and soon so shall others

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2784

Differential Revision: https://secure.phabricator.com/D5929
Merge branch 'master' of http://github.com/facebook/arcanist