Scaling bug with pull/push -v

[Robert Collins]

the bzrlib.log.show_changed_revisions api wants the full revision
history of a branch; but then does not use it.

This should be fixed - either we could pass in a decorated
revision_history that uses a generator to get access as needed, or just
pass in revision ids to show_changed_revisions and walk from there.

 affects /products/bzr
 status triaged
 importance medium

-Rob
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