Allow running `arc land` after `git svn rebase`

Summary: `git pull` may fail in git-svn after rebasing (which is a side effect of dcommit).

Test Plan:
  $ git svn rebase
  $ git log trunk..master
  $ git pull --ff-only; echo $?

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4716