[MERGE] Use Repository.get_revision_delta() in missing rather than doing the delta

The attached patch makes ``bzr missing'' use the 
Repository.get_revision_delta() function, rather than fetching trees and 
determining the delta between them. This significantly (factor > 100) improves
the performance of "bzr missing -v" for bzr-svn branches. 

Ideally the missing code should be using get_deltas_for_revisions() and 
get_revisions() but I'll leave that for some other time.

Cheers,

Jelmer
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