LibDiff: Perform diffing-algorithm in reverse order

Previously the algorithm was being performed from the start of the
string to the end, which was a little more convenient when writing
the code, but made it more annoying to be able to properly talk
about the "start" of where the changes were happening, since we
can only re-construct the changes in reverse order of the initial
traversal.

Basically, doing the initial pass in reverse lets us reconstruct
the hunks in the correct order to begin with, and not have to worry
about reversing the hunks / lines within the hunks

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