topic: check availability of obsutil.getmarkers() for portability Before this patch, topic extension causes unintentional failure with Mercurial earlier than 4.3, because obsutil.getmarkers() has been available since Mercurial 4.3 (tests for topic on mercurial-4.* branches fail, too). This breaks "minimumhgversion = '4.0'" declaration of topic extension. This patch fixes this issue in a straightforward way for simplicity on stable branch. I'm planning to centralize such portability logic in topic extension into topic/compat.py or so on default branch for efficiency at runtime, like as evolve/compat.py.

tests: add more explicit test about empty topic from publishing

convertbookmark: advertise the new command in the readme The command is in a good enough shape now.