backward diffs in knits?

[Robert Collins]

Just chatting with Martin somewhat randomly about repositories formats -
we wondered if its possible to have knits support backward diffs?

That is, rather than patching forward, have a knit be able to patch
backwards to get content. This works by having the largest text the
first, and subsequent ones generally remove content rather than adding -
making the deltas smaller and also happily reducing the read time for
the most recent text. git packfiles do this with great effect AIUI.

It seems that it would be possible to do this with knits in two
circumstances - during a hypothethical 'optimise' or 'repack' command to
recompress all ones knits (in a safe manner, with rollback etc). And
during fetch of multiple revisions, one could use backward deltas
amongst the fetched revisions, for a slightly higher cpu cost.

Just starting with support for them, and a repack, would be interesting
in our opinion.

-Rob & Martin
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