[PATCH] to help benchmark.py run on windows

Hi devs,
 I have just been looking at running the benchmarks and have got to the stage where I can
run it on windows.

This attached patch fixes three issues with the script:
 1) use of file:// when I'm sure that file:/// is correct from previous discussions on this list
 2) make sure no \ end up is repos url file://
 3) at error handler to rmtree to handle the windows read-only files.

I have removed the "which svn" command, not sure how to do this so it is only removed on
windows as I'm not really a python person.
 I hope this is useful.
 I haven't provided a log message as I don't really think it can be applied without a bit of
editing.

Cheers
Alan Wood