Added and deleted empty files were simply ignored when running rbt patch on

SVN, Mercurial, and Perforce repositories since patch does not handle empty

files.
Now, we do extra processing after running patch to manually check for and

handle any added or deleted empty files in the patch file. These empty files

are expected to have a diff in the same format as that generated by rbt post.

Ran unit tests.
Created several review requests using rbt post on SVN, Mercurial, and

  Perforce repositories with different diffs: no empty files, an added empty

  file, a deleted empty file, multiple added and deleted empty files, and

  multiple added/deleted empty and non-empty files. Ran rbt patch and

  verified that the empty files were added/deleted (via svn status, etc.).