When running as root, skip the unreadable file tests, because, well,
they're probably _not_ unreadable to root unless you've got some
crazy NSA clearance-level honoring operating system shit going on.

WD iterator: properly identify submodules

tree_iterator was only working properly for a pathlist containing
file paths. In case of directory paths, it didn't match children
which contradicts GIT_DIFF_DISABLE_PATHSPEC_MATCH and
is different from index_iterator and fs_iterator.

As a consequence head-to-index status reporting for a specific
directory did not work properly -- all files have been reported
as added.

Include additional tests.

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.

The previous commit left the comment referencing the earlier state of
the code, change it to explain the current logic. While here, change the
logic to avoid repeating the copy of the base pattern.