Minor coding style fixes
Remove incorrect swapping code from diffseq()

Here, m and n are accessed before being initialized, and so a and b are never
swapped.  Swapping a and b for computing a diff is correct (but useless) for
computing the Longest Common Subsequence, but incorrect for computing diffs, so
remove this code.
Ignore equal elements at the ends of sequences in diffseq()

This fixes the two test cases with non-optimal results.
Add another test case with non-optimal result