Change the capability for Perforce moved files support.
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.

davidt/rbtools,reviewboard/rbtools,davidt/rbtools,halvorlu/rbtools,datjwu/rbtools,reviewboard/rbtools,beol/rbtools,datjwu/rbtools,haosdent/rbtools,beol/rbtools,haosdent/rbtools,haosdent/rbtools,beol/rbtools,halvorlu/rbtools,1tush/rbtools,halvorlu/rbtools,datjwu/rbtools,reviewboard/rbtools,davidt/rbtools