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.
#require black test-repo
Black needs the real USERPROFILE in order to run on Windows
#if msys
$ USERPROFILE="$REALUSERPROFILE"
$ export USERPROFILE
#endif
$ cd $RUNTESTDIR/..
$ black --check --diff `hg files 'set:(**.py + grep("^#!.*python")) - mercurial/thirdparty/**'`