changelog-v2: add a configuration to disable rank computation

We encountered a graph where rank computation was pathologically slow. We add an option to disable this computation while this is getting fixed.

Disabling the rank computation should allow for testing other changelog-v2 feature undisturbed (like changeset based copy tracing).

I am purposely not adding a test for the new non-default code path, as this is a temporary work around of an experimental feature.