Fix an escaping issue with "svn commit"

Summary: Fixes T2438. We currently escape everything with '@', but SVN rejects that for '.'

Test Plan:
Unit tests. Performed this commit:

  $ svn st
   M      .
  A       x@123
  $ arc commit --conduit-uri=http://local.aphront.com:8080/ --revision 53

      Revision 'D53: asdf' has not been accepted. Commit this revision anyway?
      [y/N] y

  Committing 'D53: asdf'...
  Sending        .
  Adding         x@123
  Transmitting file data .
  Committed revision 37.
  Done.

I grepped for more '@' adding but couldn't find any. It's a bit tricky to grep for though, so it's possible I missed some.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran, mbishopim3

Maniphest Tasks: T2438

Differential Revision: https://secure.phabricator.com/D4703