The optimization in question could lead to an empty source and thus to
no highlighting. Checking for 'force' and the revisions is at this point
not enough since later the semantic info calculcation can be aborted, so
that a subsequent recalculation would get an empty document.

contents() itself is already optimized (uses caching).

Task-number: QTCREATORBUG-11367
Change-Id: If31a8ab0c848e4babd38104961fb9812f077a2be
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>