tests/blacklists/linux-vfat

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

# invalid filenames
test-add.t
test-init.t
test-clone.t
test-contrib.t
test-hgweb-raw.t
test-walk.t

# no sockets or fifos
test-hup.t

# no hardlinks
test-hardlinks.t
test-relink.t

# exec bit problems
test-convert-bzr-114.t
test-convert-bzr-directories.t
test-convert-bzr-merges.t
test-convert-bzr-treeroot.t
test-convert-darcs.t
test-merge-tools.t

# debugstate exec bit false positives
test-dirstate.t
test-filebranch.t
test-merge-remove.t