This patch:

  * Switches to using bzrlib directly instead of execing bzr.
  * Removes the unused get_filenames_in_revision() method (grepping through all py files in review board reveal that it is never used, only defined).
  * Adds support for revid in the diff header, output by the new bzr-diff-revid plugin (https://launchpad.net/bzr-diff-revid).

It turns out that date: revspecs, while they work most of the time, fail under some circumstances. Looking up file contents by revid (instead of by date) is faster and better supported by bzr. This patch still supports date: revspecs, so it will work with standard "bzr diff" output, but diffs with revids (created with the diff-revid plugin installed) will generally work better.

A week or so of use on our internal Review Board server.