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.
On case insensitive systems, when given a user-provided path in the
higher-level index addition functions (eg `git_index_add_bypath` /
`git_index_add_frombuffer`), examine the index to try to match the
given path to an existing directory.
Various mechanisms can cause the on-disk representation of a folder
to not match the representation in HEAD or the index - for example,
a case changing rename of some file `a/file.txt` to `A/file.txt`
will update the paths in the index, but not rename the folder on
disk.
If a user subsequently adds `a/other.txt`, then this should be stored
in the index as `A/other.txt`.
When we pass the path of a repository to `_bypath()`, we should behave
like git and stage it as a `_COMMIT` regardless of whether it is
registered a a submodule.
