Clear refs for a group after references are emitted for the group.

* A references tag for a group can appear multiple times on a page.
  Ex: en:List_of_number-one_albums_from_the_1960s_%28UK%29 uses
  {{Reflist|group=lower-alpha}} more than once, and each reflist
  tag emits references accumulated since the previous references
  output.  This requires clearing the refs for the group after it
  is emitted.

* Parsoid wasn't clearing the group's refs.  This resulted in
  duplicate ref output which in turn crashed template encapsulation
  code that relies on template ids being unique.

* This patch fixes it which fixes the crasher on
  en:List_of_number-one_albums_from_the_1960s_%28UK%29

* No change in parser test results.

Change-Id: I48ba9c3fe2249c6ef1c1bf068c22c7985fa16e8f
Merge "Clear refs for a group after references are emitted for the group."