stablerange: compute subranges from parent when possible
Unless we are a merge, we can infer the standard subranges of a range by reusing
the subranger of its parent. We update the implementation to do so.
We still needs the "old way" for merge. We move that code in a dedicated
function for the sake of simplicity.
test-drop.t
test-inhibit.t
test-simple4server.t
tests/test-simple4server-bundle2.t