tests: add an end-to-end test to show a bug in `visit_children_set`
Concretely, `rootfilesin` is completely broken with respect to
`visit_children_set` optimization.
fixbundle(){ grep -v 'saving bundle'| grep -v 'saved backup'|\ grep -v added | grep -v adding |\ grep -v "unable to find 'e' for patching"|\ grep -v "e: No such file or directory"|\ cat}