Fix arc patch adding unnecessary files for hg repos

Summary:
Arc patch was committing with -A (--addremove) which meant any random
files that were sitting around in the repo (like conflict .orig files) were
added to the commit.  The -A isn't even necessary since the hg import
adds and removes all the appropriate files for you.

Test Plan:
touch foo
arc patch --diff some-diff-id
Verified that foo was not added to the commit

Reviewers: epriestley, nh

Reviewed By: epriestley

CC: dschleimer, bos, sid0, aran, Korvin

Differential Revision: https://secure.phabricator.com/D5396