[MERGE] Accelerate no-op pull

[Aaron Bentley]

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Hi all,

This branch accelerates pull when the desired revisions are already
present in the repository.

- - It delays history retrieval until it has been shown to be needed
- - It adds a shortcut to InterRepository._walk_to_common_revisions for
  the case where the revision is already present in the repository.
- - It adds a shortcut to InterPackRepo.fetch for the case where no
  revisions need be retrieved.

This changes the time for a no-op pull of bzr.dev from 13 seconds to 2
seconds for me.  (6.5x faster)

There is no apparent effect on loom branches, which have their own pull
logic.

Aaron
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