bzr push does too many hpss calls to discover branches are diverged

[Martin Pool]

After rebasing lp:~mbp/launchpad/643223-dkim-aliases, I did `bzr push` to Launchpad. This eventually reported, correctly, that the branches had diverged, but it took 454 hpss calls and 240s to do so. We ought to be able to detect this case with just one call: read the destination tip and see if it's in the local ancestry of the local tip.

[Robert Collins (lifeless)]

There are two forms of non-divergence:
remote tip is in local ancestry: ok to push
local tip is in remote ancestry: pushing is a no-op.

Only if both are false is it diverged. It may still be a
one-round-trip thing though, with some care.

-Rob