Currently we require rootish tasks to belong to a task group that has no
more than five dependencies.

Sometimes (as in a recent Xarray workload showed to me by @dcherian)
this doesn't work because there are a few dependencies, but still far
fewer than there are tasks in the task group.  In his case there were
6000 tasks in the task group and 6 dependencies of that group, and so it
 was erroneously classified as non-rootish.

This PR proposes a change to the logic, where we accept tasks whose
groups have <1% of the number of dependencies as they have tasks in the
group.  So in Deepak's case because there are fewer than 6000 * 1% == 60
dependencies, these tasks get classified as rootish.

Future note, we're doing this dynamically now, maybe we should be
looking at data stored instead of number of tasks.

…tasks