Branch.pull() doesn't shortcut when last_revision is identical

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I'm not sure when this changed, but Branch.pull(other_branch) no longer
checks if other_branch.last_revision() == self.last_revision() =>
nothing to do.

Which means that even without changes, the fetch code is invoked, and
the graph search code is triggered. (Causing us to read the remote
revision indexes looking for the parents of the current last_revision, etc.)

I don't think it would be hard to fix this, but I guess there is some
concern about increasing the number of round trips. (It *shouldn't* as
we need to know the last_revision of the remote branch to start the pull
anyway, and we likely have the local last_revision cheaply as well.)

In my testing, this is a rather significant round-trip improvement for
no-op pulls.

 affects bzr
 status confirmed
 importance medium
 tags easy performance
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