bookmark: add an --active flag to display the active bookmark There is currently no official simple way to retrieve the current bookmark. In particular for automation. We add a `--active` flag to the `hg bookmarks` command. When set, the command display the current bookmark name if any or return 1. For now, this flag is read-only. However sensible combinations exist with `--delete`, `--rename` and `--rev` and can be implemented later.