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.
= Mercurial XXX =
== New Features ==
== Default Format Change ==
These changes affect newly created repositories (or new clones) done with
Mercurial XXX.
== New Experimental Features ==
== Bug Fixes ==
== Backwards Compatibility Changes ==
== Internal API Changes ==
== Miscellaneous ==