transaction: make checkambigfiles a private attribute
This holds instance state that is passed in at __init__ time. It
doesn't need to be exposed as part of the public interface.
Differential Revision: https://phab.mercurial-scm.org/D4628
transaction: make validator a private attribute
This is similar to releasefn. It holds state that doesn't need to be
exposed as part of the public interface.
Differential Revision: https://phab.mercurial-scm.org/D4627