Change the capability for Perforce moved files support.
Review Request #3792 — Created Jan. 26, 2013 and submitted
Change the capability for Perforce moved files support. This updates the capability check to use the new scmtools.perforce.moved_files capability going into RB 1.7.3. This is used instead of diffs.moved_files so that we don't generate a new-style Perforce diff with moved file info for older versions of Review Board that can't support it.
Ran post-review --output-diff with RB running the release-1.7.x branch. Saw the moved file info. Ran it against release-1.6.x and saw the older-style diff, indicating it wasn't using the old capability.