[Reviewers] Accept process and diffhashes

[Matt Mackall]

I've improved the accept process machinery so that accept marks now survive
trivial rebases. Details here:

https://www.mercurial-scm.org/wiki/AcceptProcess#Rebasing_and_diffhashes

Next, I'm going to add support for putting a hold on a patch that's already in
hg-committed so if you spot something that needs fixing up, you'll be able to
keep someone else from approving it while you're fixing it up or reordering
patches.

-- 
Mathematics is the supreme nostalgia of our time.