[MERGE] Significantly reducing execution time and network traffic for trivial case of `bzr missing`

[Alexander Belchenko]

Running `bzr missing http://bzrdev.bialix.com/fast-missing/`
from local mirror of fast-missing branch (i.e. branches are identical) with bzr 1.2
I get:

data received: 2135 KB
data sent: 60 KB
execution time: 191.094 seconds

With this patch I get:

data received: 5 KB
data sent: 3 KB
execution time: 7.875 seconds.
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