This change adds the rbt post logic for uploading binary files to

diffs. We start by assembling a list of the newly-created FileDiffs that

are marked as binary (using the new binary flag to the FileDiff API).

This is either done for the individual commits (when posting with commit

history) or for the full diff (when posting a squashed diff or on tools

that do not support history). We then go through and find what files we

should upload, filtering by whether it's a reviewable MIME type and the

configured file limit. We then create new file attachments for each

matching binary file.
This includes the base SCMClient flag and method definitions, but does

not implement upload for any SCMs. We have to do it per client because

we need to add new APIs to the clients to get file content (and

optionally file size) at specific revisions. Individual per-SCM

implementations will come in future changes.

Ran unit tests.
Posted a whole bunch of changes with Git and SVN repositories that

  included image files. Tested with both commit history and squashed

  diffs.