test-topic-dest: test fixes I should have put in the previous change I split a bigger change from marmoute that included some rebase support that I wanted to discuss more, but I didn't correctly commit my fixes to the tests before pushing. Oops.

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.