`strip_prefix="/"` in `mtree_mutate()` deletes everything

[mering]

We want to make all paths relative by using strip_prefix = "/". While this works just fine with pkg_tar, when using mtree_mutate() it deletes the whole file instead of just removing the / prefix from all paths.

[thesayyn]

Hmm that sounds bad, fixes are welcome, note that strip_prefix is not supposed to be identical to pkg_tar.