revlog: add a `get_revlog` method
This might seen weird, but I actually thing we have been needing this for a
long time. There is multiple object that kind of pretend being revlogs while
actually wrapping the actual revlog. Since multiple code needs to access the
actuel revlog. See documentation for more details.
Expect cleanup of various places one the current series is done.
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.