Add message when pulling to indicate from and to causal hashes.

[ChrisPenner]

Overview

Adds a message indicating where we're pulling from/to,
this will be helpful for cases where pulls are feeling really slow, I can investigate them more easily.
cc @pchiusano

Implementation notes

When pulling on a branch, show the old and new causal hashes.

Interesting/controversial decisions

Probably not the best that we need to do this at all, but more visibility into what's happening isn't a bad thing, and it makes it easy for users to reset #old if they want to undo a pull.

Test coverage

I tried it out