knit performance 1

[Robert Collins]

This is a knit performance improvement patch:

 * optimised weave-weave joins of empty target weaves
 * partially optimised weave-knit joins of empty target knits
 * ghost aware knits
 * massive reduction in ghost revision object traversals in favour of
revision graph traversals.
 * corrects an idiotic error I made that lead to ~quadratic  behaviour
during common_ancestor selection
 * reduces the revision-reads to a worst case of 2 during a diff -r
ancestor:PATH operation, I'll work on making that zero tomorrow.

This should reduce the somewhat insane performance of diff, merge,
branch at the moment.

Rob

-- 
GPG key available at: <http://www.robertcollins.net/keys.txt>.
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