[MERGE] Prevent knit corruption by checking parents

[Aaron Bentley]

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Hi all,

The optimized knit join code doesn't provide the same correctness
guarantee that the unoptimized knit codepath provides.

Specifically, if two knits disagree about what the content of a version
is, then a delta from a direct descendant of that version cannot safely
be raw-copied.  This usually means that the content of the child in the
target doesn't match the content in the source.

This patch causes a KnitTextsDiffer exception to be raised when knits
disagree about the content of a parent.

I don't believe it is useful to try to handle this problem robustly,
because if two knits disagree about the text of a version, we have a
model violation, which is a serious problem.

This does not attempt to detect format errors in the source knit.  And
if either the source or the target is already corrupt, you may still get
a corrupt target.

Aaron
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