merge with stable

obshashrange: document the experiment This should help people jump in.

obshashrange: add an option to disable local cache warming The cache still a bit too volatile an costly to update. This offer a way for developer working copy to only compute it when used.

obshashrange: re-warm less revision after reset rewarming every single revision get very expensive quickly. Instead we warm few key revision and rely on the fact this will warm enough useful subrange.