Untangle git_futils_mkdir from git_futils_mkdir_ext - the latter
assumes that we own everything beneath the base, as if it were
being called with a base of the repository or working directory,
and is tailored towards checkout and ensuring that there is no
bogosity beneath the base that must be cleaned up.

This is (at best) slow and (at worst) unsafe in the larger context
of a filesystem where we do not own things and cannot do things like
unlink symlinks that are in our way.

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

Find the first index entry matching a prefix.

We were missing tests for switching the case-sensitivity of an index
in-memory and then looking up entries in it.

This will tell us which numbers we were trying to compare, rather than
just telling us that they're different.

git_index_add_frombuffer enables now to store a memory buffer in the odb
and to store an entry in the index directly if the index is attached to a
repository.

Introduce `git_indexwriter`, to allow us to lock the index while
performing additional operations, then complete the write (or abort,
unlocking the index).

We need these directories to exist so cl_git_mkfile() can create the
files we ask it to.