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