Summary:
This adds an extension which will allow a user to specify commit 'extras' from
the command line. This will allow our repo sync script to include extra metadata
in the commits.
For instance, this is required in order to not lose git's committer info when
syncing into mercurial (which is required in order to make sure two commits with
the same content but different committers will produce a different hash in
Mercurial).
Test Plan: Added a simple test
Reviewers: pyd, lcharignon, mpm, sid0, rmcelroy
Reviewed By: rmcelroy
Differential Revision: https://phabricator.fb.com/D2027341
Signature: t1:2027341:1430255572:6866d0ba5564ca977c8fce41e55988883e15ce5b
