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.