API: Fix prop=revisions&rvdiffto= without rvprop=content

Make sure $this->section is initialized (and other stuff is checked)
when rvdiffto is used.

Bug: 64744
Change-Id: Ie6c50ad946ed5150819d004dabfe898a131a0a07
Merge "API: Fix prop=revisions&rvdiffto= without rvprop=content"