When requesting fewer history elements than we actually have, fix the
interpolation loop to not reverse the returned elements; this already does not
happen if we request more elements than available, which led to the returned
list order depending on the history size.
Now the first array element is always the most recent one. Update documentation
accordingly.
Add test case to reproduce the problem. We now add three sample points to be
able to request a subset and still assert its correct order, and make the
charge values be further apart to ensure correct interpolation.
https://bugs.freedesktop.org/show_bug.cgi?id=68384