commands: disallow 'hg debugobsolete --index --rev <smth>' A bug in the original --index implementation. The goal of --index is to allow unique obsmarker identification that would be consistent between invocations of this command in the unchanged repo. Further goal is to use this index to delete arbitrary obsmarkers. So calling --index together with --rev would cause obsmarker indices to be different than just calling --index. This is not desired and current pattern for getting the index of an interesting obsmarker is: `$ hg debugobsolete --index | grep <interesting hash>`. It would clearly be better if we could somehow compute a hash of an obsmarker and use it to identify the one we want to delete, but it seems a bit too heavy for our current goals, so we can do this later if we want.