prune: do not prune beyond an optional base directory, and add a test

prune: do not prune beyond an optional base directory, and add a test

Previously, prune("wiki/srcdir/sandbox/test.mdwn") could delete srcdir
or even wiki, if they happened to be empty. This is rarely what you
want: there's usually some base directory (destdir, srcdir, transientdir
or another subdirectory of wikistatedir) beyond which you do not want to
delete.