[MERGE] Simple optimization of common annotate case

[John Arbash Meinel]

I've been trying to improve 'bzr annotate' performance. It turns out 
there are 2 pieces to it. The first is that we still pay for 
"get_revision_id_to_revno_map()" which is slower with packs as it is a 
whole-history operation. If you do 'bzr annotate --show-ids' then it 
avoids that code, so I started there.

What I found was that "reannotate()" was actually the big slowdown. 
Something about how it processed each line was being slow. The actual 
"_reannotate" code which goes and does a diff barely showed up.

So this code just switches around 2 things:

1) reannotate() and _reannotate() work on lists rather than being 
generator functions. Under lsprof this was quite a bit faster, though 
that might just be lsprof overhead.

2) Special case when there are only 2 parents. You can do simple == and 
!= comparisons without having to create a set for each line, etc. It 
also allows us to share the tuples

According to '--lsprof' this dropped the time spent directly in 
reannotate from 8s down to 1s. And manual 'time bzr annotate --show-ids' 
dropped from 13s to 8s.

The tests still pass, and the logic is pretty much the same, just 
spelled out, rather than using a set() to do the logic. Going by lsprof, 
the big expense was certainly the last loop, I don't know if it was the 
zip() or all of the if checks.

John
=:->
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