topic: fix changing topics of commits that have an amend_source Includes a test for this specific case, but we should probably spend more time squinting at the interactions between the current topic wrapper function and commitctx for memctx instances.

topic: remove useless call to c.parents()

topic: fix up change logic a little and add debug logging

merge: use topic to pick default destination If the topic have multiple heads, reduce that. Otherwise, pick the branch we are on.