Add a capability to indicate support for Perforce moved files.

1.7.2 introduced moved file support for Perforce, but there wasn't a
good way for RBTools to really detect this that retained compatibility
with previous versions of Review Board. The reason is that we relied
upon the existing general diffs.moved_files capability, causing RBTools
to generate diffs for older releases that ended up being incompatible.

This introduces a scmtools.perforce.moved_files cap that can be used
instead. 1.7.3 will be the minimum version of Review Board that will
support moved files for Perforce with RBTools.

Checked that the capability was in the payload. Made sure no existing
tests broke.