headssummary: ensure all returned lists are sorted This is a simple step that will help to keep a stable output in coming refactoring.

discovery: also use lists for the returns of '_oldheadssummary' The '_headssummary' function is documenting and using list objects in its return. We now use them in _oldheadssummary too for consistency. This does not affect any usages of these values.

test: add a push race case where the updated head is obsoleted This is the mirror of the previously added case. We check the case where the racing-push obsoletes a head while the raced-push updates that same head.

test: add a push race case where obsoleted head is updated We check the case where the raced-push obsoletes a head while the racing-push updates that same head.