[MERGE] allow recovering bad texts from knits

[Robert Collins]

When a line delta has a bad basis - which baz imports can create- it can
be useful to be able to inspect the result of the text delta application
even when the sha1 is wrong and it fails validation.

This patch adds a attribute and specific exception allowing this to be
done.

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