On case insensitive platforms, allow `git_index_add` to provide a new
path for an existing index entry.  Previously, we would maintain the
case in an index entry without the ability to change it (except by
removing an entry and re-adding it.)

Higher-level functions (like `git_index_add_bypath` and
`git_index_add_frombuffers`) continue to keep the old path for easier
usage.

This was not converted when we converted the rest, so do it now.

Clay is the name of a programming language on the makings, and we want
to avoid confusions. Sorry for the huge diff!

Thanks to Emeric.