[MERGE] Refactor bzrlib.osutils._walkdirs_utf8 to aid API migration in future.

[Robert Collins]

This patch refactors the guts of _walkdirs_utf8 to be more modular. 

In my testing its no slower than trunk (3ms, the expected overhead of a
call-per-dir on mozilla trees is below noise level). I believe my
changes to the win32 walker are correct - that is that the win32 feature
tests will pass and it will run well on win32; I would appreciate a
win32 developer (two are cc'd to this) confirming that this is the case
before I submit it to PQM [assuming it gets through review].

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