[PATCH 1 of 2] split: new extension to split changesets

[Jun Wu]

Excerpts from Martin von Zweigbergk's message of 2017-06-25 00:26:32 -0700:
> Why is a2 still here? I would expect it to get stripped.

Sorry, I missed this one (I probably want to do some email client changes to
make sure I won't miss comments).

It's a bug and fixed in V2.

> Could you add another test for "Split a non-head with obsolete
> descendant"? I'm curious what the behavior would be. If the descendant
> had a successor, would we get divergence through the rebase?

That feels like rebase's responsible to do it right. I'll try to fix
rebase's code instead. But that's depending on a lot of things already so
I'll probably defer sending split patches until rebase is good enough to
use.