[merge][1.6][#234748] fix KnitCorrupt error in check on bzr.dev

[Martin Pool]

Here's an updated version with review feedback from Robert and
including his related changes to Knit.add_lines with matching blocks
specified.

Robert suggests that John might want to scrutinize this for knit
extraction performance.  I am sure we are doing more copying of data
in here than the minimum necessary, but not because of this change.

-- 
Martin <http://launchpad.net/~mbp/>
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