index: canonicalize directory case when adding

New feature: add the ablility to iterate through a directory in index

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`.

Use a hashmap for path-based lookups in the index

Find the first index entry matching a prefix.

This should let us see more clearly what we're doing and avoid the ugly
'if' we need every time we want to interact with the map.

The hash table allows quick lookup of specific paths, while we use the
vector for enumeration.