(Bug 60120) Reset pipeline state before parsing original wikitext

• This bug was exposed by commit 8a7a5cb1 (since the _rtselser diff is not present before 8a7a5cb1).

• SelectiveSerializer needs to reset parser pipeline state before using it to parse original wikitext since it would change cite ids assigned to references which then shows up as dom-diffs in the /_rtselser/ endpoint.

Change-Id: I01a3c3d0bda578cfbc5489290b3cd52e3ecd1826