API: Don't apply size limit to action=compare

API: Don't apply size limit to action=compare

Some diffs can be really huge. Since ApiComparePages is intended to
return only the diff and some small metadata, bypass $wgAPIMaxResultSize
to allow the diff to be returned even if it's insanely huge.

Really insanely huge diffs might still make PHP run out of memory, even
before it gets the chance to return the diff to the API. That is outside
the scope of this change.

Bug: T184412
Change-Id: I755417f573c3caf85031d03b262aa55bd8cf36d7