sparse-revlog: introduce native (C) implementation of slicechunktodensity
This is a C implementation of `_slicechunktodensity` in the
`mercurial/revlogutils/deltas.py` file.
The algorithm involves a lot of integer manipulation and low-level access to
index data. Having a C implementation of it raises a large performance
improvement. See later changeset in this series for details.
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state