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.