escape files properly in arc commit

Summary: turns out retina has files like image.png and image@2x.png. The latter breaks since '@' is a special command telling svn to "look for image at revision 2x.png" -- nonsensical garbage. If we add it to the end every time this error goes way.

Test Plan: touch 2@2.png; svn add '2@2@'; arc diff; <fill out form, accept commit>; arc commit -- observe commit working

Reviewers: epriestley

Reviewed By: epriestley

CC: mbishopim3, aran, Korvin

Maniphest Tasks: T1999

Differential Revision: https://secure.phabricator.com/D3845