[MERGE] Accelerate no-op pull

[Robert Collins]

On Tue, 2008-04-29 at 20:44 -0400, Aaron Bentley wrote:
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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.

get_graph() shouldn't be doing any intrinsic history retrieval, so I'm
not sure how it delays history retrival. The locking improvements look
significant and useful.

However, do all tests still pass? It looks to me like this will no
longer error on a missing revision in the source (which is part of the
current contract) and why the current code didn't just check all
requested items were in the target. If all tests pass:

bb:approve.

-Rob
-- 
GPG key available at: <http://www.robertcollins.net/keys.txt>.
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