First of all, it ran `is_btrfs "$chrootdir"` to decide if it was on
btrfs, but $chrootdir wasn't defined locally; it just happens to work
because $chrootdir was defined in main().  (I noticed this because in
Parabola, it is called differently, so $chrootdir was empty).

So I was tempted to just change it to `is_btrfs "$copydir"`, but if
$copydir is just a regular directory on a btrfs filesystem, then it
It would leave much of $copydir intact.  What we really care about is
if $copydir is a btrfs subvolume; which we can check by combining the
is_btrfs check with inspecting the inum of the directory.

I put this combined check in lib/archroot.sh:is_subvolume.

https://lists.archlinux.org/pipermail/arch-projects/2013-September/003901.html