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
adding da/foo
adding foo
% manifest
200 Script output follows
drwxr-xr-x da
-rw-r--r-- 4 foo
200 Script output follows
-rw-r--r-- 4 foo