Performance regression on stable

[Matt Mackall]

This changeset dropped perf on the hg repo itself rather noticeably. I
just spent an entire evening rewriting util.chunkbuffer and wondering
why it was consistently slower than 2.4 even though all of my
microbenchmarks looked better. Going to back this out for now, the next
attempt belongs on the default branch.

changeset:   17952:54cedee86e51
branch:      stable
user:        Durham Goode <durham at fb.com>
date:        Fri Nov 16 15:39:12 2012 -0800
summary:     commit: increase perf by avoiding checks against entire
repo subsets

Before:

$ time hgs clone --pull -U hg hg2
requesting all changes
adding changesets
adding manifests                                                                
adding file changes                                                             
added 17963 changesets with 35091 changes to 2171 files                         

real	0m12.488s
user	0m11.783s
sys	0m0.610s

After:

$ rm -rf hg2; time hgs clone --pull -U hg hg2
requesting all changes
adding changesets
adding manifests
adding file changes                                                             
added 17963 changesets with 35091 changes to 2171 files                         

real	0m14.748s
user	0m13.936s
sys	0m0.713s

-- 
Mathematics is the supreme nostalgia of our time.