Levenshtein maximum distance is greater than length of both strings

```

What steps will reproduce the problem?
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Compute the levenshtein distance between "709113544" and "1555855732":
var dmp = new diff_match_patch();
var diffs = dmp.diff_main("709113544", "1555855732");
var distance = dmp.diff_levenshtein(diffs);

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What is the expected output?
10 (that's the length of the bigger string)
What do you see instead?
14

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What version of the product are you using? On what operating system?
Javascript version 20121119. Running in node.js, also reproduced in chrome:
http://plnkr.co/Qgi9q4IyWmguK5mIAMxy
```

Original issue reported on code.google.com by `andersly...@gmail.com` on 22 May 2013 at 7:28