tests/test-gendoc-zh_TW.t

index: embed nodetree in index object to avoid reference cycle

Since the index has a reference to a nodetree and the nodetree has a
reference back to the index, there is a reference cycle, so the index
(and its nodetree) can never be freed. This patch fixes that by making
"nodetree" a plan C struct that the index can embed, and also
introduces a new "nodetreeObject" that is a Python type wrapping the
nodetree struct.

Thanks to Yuya for noticing this and for suggesting the solution.

All tests passed on the first attempt once it compiled (I guess C is
like Haskell in this regard?).

Differential Revision: https://phab.mercurial-scm.org/D4372

#require docutils gettext

  $ $TESTDIR/check-gendoc zh_TW
  checking for parse errors