Redirect page diffs to Special:MobileDiff

When diff parameter is passed
Make Special:MobileDiff show diffs other than current to previous
by allowing subpage to be of form <rev1>...<rev2>

Deal with the fact diff can be prev or next.

Change-Id: Id0c4b9b33e98f5caecdceaff48d1063bddea8339
Merge "Redirect page diffs to Special:MobileDiff"