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.
Set up
$ hg init repo
$ cd repo
Try to import an empty patch
$ hg import --no-commit - <<EOF
> EOF
applying patch from stdin
abort: stdin: no diffs found
[255]
No dirstate backups are left behind
$ ls .hg/dirstate* | sort
.hg/dirstate