speeding up commits

[duchier at ps.uni-sb.de]

Here is a quick report on my progress speeding up commits.  My test
example is a 1 line change, in 1 file, of a project with 1048
entries.


      HACK                                    | SECONDS
----------------------------------------------+--------
[1] = stock bzr                               | 35
[2] = [1] + "prelude" optimization            |  7.2
[3] = [2] + "lifeless's BzrCherrypickMetada"  |  6.3
[4] = [3] + caching "inventory" weave         |  5
----------------------------------------------+--------


I am a little disappointed that [3] doesn't provide greater savings.
[4] was a hack to get around the fact that the PassThroughTransaction
object used during commits doesn't cache anything.

I am not especially interested in the "transactional" aspects here,
but we really need a cache that works during commit.

Cheers,

--Denys