Cover incomplete revision case in RenderedRevision.

Cover incomplete revision case in RenderedRevision.

This introduces a check that allows magic words like {{REVISIONUSER}}
to still work even when an incomplete fake RevisonRecord is provided to
RevisionRenderer. In such a case, the revision is loaded from the database
if needed. This lets old code that only has access to a Content object for
rendering use that Content object in a fake RevisionRecord, instead of
having to load the full revision from the database in all cases.

Bug: T174035
Change-Id: I658eab97a8282b8943baf7968f3256da35789ec1