SI-7729 Does Par-Test work?  Absolutely!

Absolutize paths where possible; cherry-pick the fix for #ifdiff check files; and zap the log file before starting a run.

It also touches the partest script with the new voudou and to cd -P > /dev/null, to ignore the extra echo if CDPATH matches.

Why isn't this a problem with scalac? Presumably I never run it from that location.

In fact,
```
apm@mara:~/projects/snytt$ build/pack/bin/scala
Error: Could not find or load main class .home.apm.projects.snytt.build.pack.lib.diffutils.jar
```
Ha, I don't even know what that means.

My usual aliases have absolute paths.

Almost forgot:

Review by @paulp