`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`.