Regtest script improvements:

Regtest script improvements:

- Allow to customize the length of shortened strings.
- When filtering through js, don't filter out header rows.
- Output an header row at every change of directory.
- Add the prefix to the filename (e.g. input/regression).

[Julien Rioux, 2011-12-18 16:51:59]

Some general improvements in the way the regtest comparison page is generated,
including displaying the path of the input file relative to the top source
directory.