head-checking: backport fix from 7d5455b988ec See 7d5455b988ec for details. Having a version in the evolve repository allow us to check the behavior when topic is enabled. It will also become the base of further head checking fix and topic compatibility.

head-checking: filter out obsolete heads when checking for new heads This does not affect any tests right now, but this will be useful for the next changeset. This is also the start of a more aggressive removal of obsolete heads in places where it does not make sense to take them into account.