Add a differential.abandon Conduit method.

Summary:
Brazenly copied from differential.close. Ulterior motive
is to be able to automate discarding of useless commits from
arcanist testing.

Test Plan:
Called method in phabricator sandbox. Revisions were
successfully abandoned.

Reviewers: epriestley

Reviewed By: epriestley

CC: nh, aran, Korvin

Differential Revision: https://secure.phabricator.com/D2928