Win32 Reserved names: don't reserve names outside the working directory

Check that the repository directory is beneath the workdir before
adding it to the list of reserved paths.  If it is not, then there
is no possibility of checking out files into it, and it should not
be a reserved word.

This is a particular problem with submodules where the repo directory
may be in the super's .git directory.

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.

Remove run-time configuration settings from submodules

This is something we do on re-init but not when opening a
repository. This hasn't particularly mattered up to now as the version
has been 0 ever since the first release of git, but the times, they're
a-changing and we will soon see version 1 in the wild. We need to make
sure we don't open those.

git will assume the repository format version is 0 if the value is not
there. Do the same.

Having this cache and giving them out goes against our multithreading
guarantees and it makes it impossible to use submodules in a
multi-threaded environment, as any thread can ask for a refresh which
may reallocate some string in the submodule struct which we've accessed
in a different one via a getter.

This makes the submodules behave more like remotes, where each object is
created upon request and not shared except explicitly by the user. This
means that some tests won't pass yet, as they assume they can affect the
submodule objects in the cache and that will affect later operations.