[RFC] make checkouts as much like bound branches as possible

[Aaron Bentley]

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True checkouts and bound branches are very similar in the UI, and we've
tried to have them exhibit the same behavior wherever it makes sense.

But since they are implemented quite differently, they exhibit different
bugs and require different code paths to get the same behavior.

I'd like to propose that checkouts have a special proxy branch, a
CheckoutBranch, whose revision-history operations differ from the
proxied branch.  CheckoutBranch.get_bound_location and
CheckoutBranch.get_master_branch would both refer to the proxied branch.
~ CheckoutBranch.unbind and CheckoutBranch.bind would throw.

Aside from this, all other operations would pass through to the proxied
branch.

This would mean
- - fewer codepaths
- - more behavioral consistency between bound branches and checkouts, more
easily
- - less confusion for developers about how bound branches/checkouts are
supposed to behave.
- - all code that expects revision-history to be the local tree's revision
history (e.g. the revision spec code) will become correct
- - James Blackwell can truthfully say 'A checkout is a branch'.
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