project-disk-use: follow du command line args if possible

project-disk-use: follow du command line args if possible

On a deployment where bind mounts are not available and
chroot sshd support is being used, the $Girocco::Config::reporoot
might end up being a symbolic link to the actual location
of the repositories inside the chroot.

In this case we need to follow the link when getting disk
use information.  Although not POSIX, the -H option to
du seems to be widely supported, so use it if it's
available.

In the future we may use an alternate method to resolve
the symlink that does not require -H.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>