https://tangled.org does exactly what you want :)

have a longer write up here: https://blog.tangled.org/stacking but we have "interdiffs", to view a delta from previous review. pull-requests advance in the form of immutable rounds much like the patch workflow on email.

we have been interdiffing and stacking for a while on to dogfood, sample PR: https://tangled.org/tangled.org/core/pulls/1265/round/1?diff...