OutputPage->getJSVars: Maintain symmetry between JS & PHP vars

OutputPage->getJSVars: Maintain symmetry between JS & PHP vars

The function is more difficult to read than it need be by dint of some things
two names. I can't be the only one who finds 'wgCurRevisionId' => $latestRevID
ugly, right? Anyhow, lots of JS code depends on the JS variable names, whereas
the PHP variables exist only in the scope of that method, so it's clear that
it's the PHP names that should be brought in line.

Change-Id: Ibddd5d2fc8d75e0ade18ff3433714d52605f2911