This change makes two improvements to the Perforce client backend in

RBTools. First, this implements the new get_file_content and

get_file_size APIs for use with uploading binary files to diffs. Second,

this plumbs through the tip revision for each file to the diff method so

that we can include it in the file headers. This was important when

posting submitted changesets so that we would have revision information

for the modified file as well as the original.

Posted a variety of submitted and pending changesets, including binary

  files.
Verified that the revision information for modified files was present

  in the diff headers when posting submitted changesets.
Ran unit tests.