transaction: make file a private attribute
This holds a file handle for the journal file. This file handle
should not be touched outside the journal class and doesn't
belong on the public interface.
Differential Revision: https://phab.mercurial-scm.org/D4625
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.