[MERGE] clean up option.py and branch.py imports

[Michael Hudson]

So the story of this branch is that I noticed in a traceback that 
option.py lazily imported bzrlib.log then promptly used it at module 
level, which is pointless.  So I removed that, and noticed that some 
other imports were also pointlessly lazy of unused, so I fixed them, so 
I picked another module to work on -- branch.py -- and found a bunch 
more work to do there -- changes so the bzrlib.errors import can be 
properly lazy, and some more unused imports.

As it looks like doing this for all of bzrlib looks like quite a bit of 
work, I thought I'd check here that the work would be acceptable by 
submitting what I've done so far for review.

I guess in the end this should shave a couple of percentage points off 
startup time, but certainly nothing is measurable yet.

Cheers,
mwh
PS: I'll see if I can hack up an import checker that undestands lazy 
imports one of these days...
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