[RFC] Removing hash prefix in storage vastly improves performance

[John Arbash Meinel]

The attached patch adds a very simple new knit repository format. The
only change is that it doesn't use the hash prefixes when figuring out
where a given file id is stored, it stores everything in one directory.

Originally, I was suspicious that this would help, but I was seeing that
25+% of the time spent committing a new kernel was just creating the
knit index header. So I gave it a shot. And what I found was that:

time bzr.dev/bzr -q commit -m "first"
8:36.95

time no-knit-hash-prefix/bzr -q commit -m "first"
4:12.68

Yep, that's right. 2x faster. Just as some comparison points:
	
 1:21.56 time bzr-0.8/bzr add
11:03.12 time bzr-0.8/bzr -q commit -m "first"

   11.31 time bzr-0.9/bzr add
 7:51.00 time bzr-0.9/bzr -q commit -m "first"

    8.62 time bzr.dev/bzr add
 7:19.14 time bzr.dev/bzr -q commit -m "first"
 8:34.66 time bzr.dev/bzr commit -m "noisy"

    8.80 time no-knit-hash-prefix/bzr add
 4:43.66 time no-knit-hash-prefix/bzr -q commit -m "first"
 5:34.66 time no-knit-hash-prefix/bzr commit -m "noisy"

I'm also working on another patch to make commit faster without using
'--quiet'. It seems 'note()' is quite a bit slower than mutter(). Also,
there seems to be a bug where we are clearing the progress bar during
commit too often. You can really tell if you use 'bzr -q commit'.

This would be pretty easy to phase into an 0.10 release, since it really
is a small change. Because it is a repository format change, we may want
to just have it available, but not the default for the next release.

John
=:->

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