Fix another SVN issue with "@" files

Summary: This chunk of code is kind of iffy and not really correct, but make it not fail, at least.

Test Plan:
  - Added a file named `swamp@2x.jpg` to a working copy.
  - Used `svn propedit svn:mime-type swamp@2x.jpg@` to incorrectly set its mime type to `text/plain`.
  - Ran `arc diff`.
  - Saw `arc diff` "correctly" change its mime-type to a binary mime type. This isn't really correct, but doing it successfully is better than throwing an exception.

Reviewers: mbishopim3, chad

Reviewed By: mbishopim3

CC: aran

Differential Revision: https://secure.phabricator.com/D4998