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.
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.
There are a number of tests that modify the global or system
search paths during the tests. This adds a helper function to
make it easier to restore those paths and makes sure that they
are getting restored in a manner that preserves test isolation.
