Fix regression in r27416 -- {{REVISIONID}} was broken on save since a parsed copy...

Fix regression in r27416 -- {{REVISIONID}} was broken on save since a parsed copy from before revision-insert was used to insert into parser cache.
The parser now sets a "vary-revision" flag on the ParserOutput when using {{REVISIONID}}; Article then reparses the page after insertion if and only if required.