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
$ hg init
$ echo a > a
$ hg ci -Am t
adding a
$ hg mv a b
$ hg ci -Am t1
$ hg debugrename b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3
$ hg mv b a
$ hg ci -Am t2
$ hg debugrename a
a renamed from b:37d9b5d994eab34eda9c16b195ace52c7b129980
$ hg debugrename --rev 1 b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3