[MERGE/RFC] faster send

[Ian Clatworthy]

Looking into the performance of "bzr send -o xxx" on chk
repositories today, I noticed it was walking all of history
twice (repo.get_ancestry()) and then calculating the difference
from there. We now have a special method (graph.find_difference())
for doing this more efficiently. This patch changes the
bundle serializer code to use it. Benchmarking the bundling of
this patch, the time drops from 5.5 secs (bzr.dev) to 2.4 secs
(bzr.dev + this patch).

I should mention that Robert suggested on IRC that
graph.find_difference() has bugs and that these were documented
in the tests or doc. I couldn't find any evidence of this
and all the send tests pass. I can say that find_difference()
is used by missing and that, at a glance, none of the bugs for
missing indicate a problem along these lines. Even so, I'm
marking this as RFC until this concern is better understood.

Ian C.
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