stack: introduce a rich stack object The logic around stack object become more and more complex. We great and object to gather and abstract it.

tests: add tests for `hg stack` showing surprising behavior with gaps The test will help us to analyse in next commit if the things are fixed or not.

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.

convertbookmark: properly convert stacked bookmarks We ignores pure descendant when looking for revision to convert. This adds support for cases were a linear stack contains multiple bookmark (or un-bookmarked descendants exists). In this case, some changeset are left orphan, but we'll consider the issue as minor for now.

convertbookmark: perform all actions at the end This will make the whole operation more deterministic and robust.