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.

converbookmark: split target computation from actual update This will allow for further refactoring helping consistency.

convertbookmark: extract the revset into a module level constant This will make it easier to read and modify the query.

convertbookmark: properly exclude public changeset Public changeset are immutable and cannot have a topic anyway.

test: add a new root changeset to all debugconvertbookmark test case We'll make this root changeset public to check the debugconvertbookmark logic in that regards. It is added in its own changeset since it massively impact all output.