Fix bug in diff_commonOverlap.
Fix bug in diff_commonOverlap.

This was an error introduced in the STL port.

Compare to line 634 in the original Qt source, which has
'text1_trunc.right(length) == text2_trunc.left(length)'.

https://code.google.com/p/google-diff-match-patch/source/browse/trunk/cpp/diff_match_patch.cpp#634

leutloff/diff-match-patch-cpp-stl,pombredanne/diff-match-patch-cpp-stl,bgrainger/diff-match-patch-cpp-stl