Update to retro 0.9.0.

This changes max-revision into revision-range, so that we can forego
the POSITIVE_INFINITY malarkey.
Revert "Remove test that asserts implementation-details of old implementation"

This reverts commit a28b737d08e7df4df7f34d6c2600e7ac3681764d.
Make merged not take named layers
Started on the double-wrapped merge-layer strategy