topics: add a new debugconvertbookmark command to convert bookmarks to topics This new command, aliased to debugcb converts bookmarks to topics. This command either accept name of a bookmark using flag '-b' or '--all'. If a bookmark is passed using '-b', the whole stack defined by that bookmark is converted to topics. If '--all' is passed, the above thing is repeated for every bookmark in the repository. If the revset which we are using unable to decides revisions of the stack and return an expty set, in that cases we are not deleting the bookmark as we are not writing any topics of the same name as that of bookmark. Also currently if a changeset has two bookmarks, than there is buggy behaviour. So these are known lackings in the current implementation which will be fixed in upcoming patches. This also adds tests for the same.