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.
cat >> $HGRCPATH <<EOF
[extensions]
narrow=
[ui]
ssh=python "$TESTDIR/dummyssh"
[experimental]
changegroup3 = True
EOF