Fix the unit test for Perforce moved files.

This was checking the old, global "are moved files supported in diffs"
capability and not the new "Does Perforce support moved files" one.
Fix treating an SVN repo as Perforce by mistake.

This changes the order in which we check for repositories. Perforce was
coming before Subversion, but Perforce would give us a false-positive
due to the fact that it says we have a valid repository so long as
P4PORT is set to something real.

This should be fixed in a more bullet-proof way later, but for now, this
at least makes it possible to use both Subversion and Perforce
individually.

Reviewed at http://reviews.reviewboard.org/r/3525/