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.