sparse-revlog: add a `trim_endidx` function in C
We are about to implement a native version of `slicechunktodensity`. For
clarity, we introduce the helper functions first.
This function implement a subpart of the python function `_trimchunk` in
`mercurial/revlogutils/deltas.py`. Handling of actual Python objects is left
to the caller function.
[workspace]
members = ["hgcli", "hg-core", "hg-direct-ffi"]
exclude = ["chg"]