bug: new way to make uncopiable circular symlinks

[StefanKarpinski]

As a file tree:

.:
total 0
drwxr-xr-x 3 stefan staff 96 Oct 26 12:03 bar/
drwxr-xr-x 3 stefan staff 96 Oct 26 12:03 foo/

./bar:
total 0
lrwxr-xr-x 1 stefan staff 6 Oct 26 12:03 foo -> ../foo/

./foo:
total 0
lrwxr-xr-x 1 stefan staff 6 Oct 26 12:03 bar -> ../bar/

As a tarball:

julia> dir = Tar.list(tarball)
2-element Array{Tar.Header,1}:
 Tar.Header("bar/foo", :symlink, 0o755, 0, "../foo")
 Tar.Header("foo/bar", :symlink, 0o755, 0, "../bar")

Indeed, if one tries to extract this with Tar.extract(tarball, copy_symlinks = true) then it hangs.