topic: check that repo.currenttopic is set before using it as an argument repo.currenttopic can be an empty string, which means there's no current topic set. This fact can be used to short-circuit some checks. Also, this avoids passing an empty string as an argument to topic() revset.

test-drop.t
test-inhibit.t
test-simple4server.t
tests/test-simple4server-bundle2.t