`git_futils_mkdir` does not blindly call `git_futils_mkdir_relative`.

`git_futils_mkdir_relative` is used when you have some base directory
and want to create some path inside of it, potentially removing blocking
symlinks and files in the process.  This is not suitable for a general
recursive mkdir within the filesystem.

Instead, when `mkdir` is being recursive, locate the first existent
parent directory and use that as the base for `mkdir_relative`.

In `mkdir` and `mkdir_r`, ensure that we don't try to remove symlinks
that are in our way.

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.

Our mkdir helper was failing is a parent directory was not
accessible even if the child directory could be created.
This changes the helper to keep trying child directories
even when the parent is unwritable.