Fix various SVN escaping issues in arc patch

Summary: Followup to D4703. When we give paths to `svn`, we need to escape them if they contain an `@`.

Test Plan:
Created a patch full of modifications to files with `@` in their names, and applied it:

  $ arc patch --diff 192
  A         A@2xcopy2
  A         A@2xcopy
  D         A@2x
   OKAY  Successfully applied patch to the working copy.

Reviewers: chad, mbishopim3

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D4977