[MERGE] Quicker initial commit

[Ian Clatworthy]

This patch contains some changes that improve the speed of an initial
commit. In particular:

* files are no longer SHAed twice
* we no longer look up the path when we already know it.

Against the current bzr.dev, the impact isn't high (~ 5%). But after the
low hanging performance fruit are plucked - namely not generating output
thrown away (see the separate 3 noise levels patch) and Robert's
(pending) better iterator, the impact of these things becomes larger
percentage wise. As these changes are independent of those other things,
I'm submitting them separately for review.

Ian C.
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