(Bug 49555) Support nested refs

* After trying various hacks, came up with a relatively simple
  fix/hack to support nested refs.

  The fix looks for {{#tag:ref..}} and short-circuits full
  pipeline expansion and converts that to an extension tag
  in place.

* Tested with the following wikitext which parses and RTs correctly

A <ref name='foo' />
B {{#tag:ref|nested ref <ref>bar</ref> |name=foo}}
<references />

* Also tested on en:Fomitiporia_ellipsoidea from the bug report
  and verified correct parse and round tripping.

* Verified that the nested ref in <ref> foo <ref>bar</ref> </ref>
  continues to be parsed as plain text.

* No change in parser test results -- have to make another
  round of updates to parser tests.

Change-Id: I43bb8b710bd10a9ddbea27818ff8aaf97ddb3fdc
Merge "(Bug 49555) Support nested refs"