tests/test-issue433

lazyindex: speed up __getitem__

This function is fairly performance sensitive, so we make a couple
ugly tweaks:

- keep all entries packed so we needn't test entry types
- fold index lookup/load into unpack call to eliminate
  local variable setting

#!/bin/sh
# http://www.selenic.com/mercurial/bts/issue433

hg init a
cd a
echo a > a
hg commit -Ama

hg parents -r 0 doesnotexist
true