(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
Merge "(Bug 60120) Reset pipeline state before parsing original wikitext"