Improve compatibility with TemplateSandbox

Use $title->exists() to see if a title exists, and use
$parser->fetchLatestRevisionOfTitle() when available, so that
TemplateSandbox works with title.exists and title.getContent().

Bug: 70495
Change-Id: I732da9daccdc35b11d726818c3a7c81f5e810a32
Merge "Improve compatibility with TemplateSandbox"