Octopus Merge Support

This will be moderately complicated, as we'll need to synthesize phony
changeset entries to explode the octopus into "revisions" that only
have two parents each. For today, we can probably just do something like

aaaaaaaaaaaaaaaaaaXX{20 bytes of exploded node's hex sha}

where XX is a counter (so we could have as many as 255 parents in a
git commit - more than I think we'd ever see.) That means that we can
install some check in this extension to disallow checking out or
otherwise interacting with the aaaaaaaaaaaaaaaaaa revisions.

Interface Creation

We at least need an interface definition for changelog in core that
this extension can satisfy, and again for basicstore.

Reason About Locking

We should spend some time thinking hard about locking, especially on
.git/index etc. We're probably adequately locking the git
repository, but may not have enough locking correctness in places
where hg does locking that git isn't aware of (notably the working
copy, which I believe Git does not lock.)

Clean up requirements

Right now (for historical reasons, mainly) hgext.git uses a
.hg/this-is-git file to detect repositories that should be treated as
git. We should look in the .hg/requires for the "git" requirement
instead (we already set this requirement, so it's mostly keying off
that instead of using an empty file.)