transaction: make releasefn a private attribute
This is a handle on a callable that is called when the journal
is closed. The value is specified at __init__ time. It doesn't
need to be exposed on the public interface. So mark it as private.
Differential Revision: https://phab.mercurial-scm.org/D4626
graft c-ext
graft zstd
graft tests
include make_cffi.py
include setup_zstd.py
include zstd.c
include LICENSE