Allow posting filenames of retina images with svn.

When adding or changing files whose filenames include '@' (such as
image@2x.png), we need to clarify in our 'svn info' command line that the '@'
in the filename isn't a revision indicator. This is done by adding an extra '@'
at the end of the filename.

Testing done:
Added a file called image@2x.png and ran 'rbt post --debug'. Saw that there
were no longer a bunch of errors running 'svn info'.

Fixes bug 2702.

Reviewed at https://reviews.reviewboard.org/r/5477/