[MERGE] Faster diff on historical data

[Lukáš Lalinský]

Hi,

currently when doing something like `bzr diff -r10..11`, bzr extracts
the inventory from knit for both revision and then does the same for
each modified file. People usually use such diffs only on nearby
revisions, so the knit extraction involves a lot of duplicated work,
e.g. instead of taking the file at revision 10 and applying one delta,
it takes revision 1 and unpacks/applies 10 deltas.

The attached patch lets it to extract both files in one go. It isn't a
big win, but it makes diff -r before:x..x on bzr.dev in my repository
about 0.1 seconds faster (~10% of the total diff time).

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