Par-Test split checks work again

Check files split into conditional blocks with partest flags
have been broken since the new diff regime.

For some reason, no one noticed.

The clever scheme to "filter the diff" instead of just filtering
the check file is abandoned as futile and unnecessary.

Fix java6 checkfile for ifdiff fix.
SI-7729 Does Par-Test work?  Absolutely!

Testicolor just passed file arguments through; this commit does
either toAbsolute or getAbsoluteFile (depending on the File API
impedance mismatch) at ConsoleRunner (command line) and Runner
(ant supplies absolute paths, but in case sbt runner doesn't,
and who knows). It also sprinkles toAbsolute internally, where
it may be redundant. Do I mean redundant or redolent?

I noticed that my branch from last year relativized to test root
always; so this is one of those choices one commits to.

The point of breakage in this case was, if a path is not absolute,
use test root / path, which only works (of course) if the relative
path is relative to test root, which is no longer the case.

Because all paths are now calculated with respect to the test file
path, the "out" dir would change its relative path depending on
the invocation.
Log file is zapped before test run

Not all test configurations handle an existing log file.

In particular, kicking off javac will append to the log file
and always fail the test.

Don't even ask how I know that.
SI-7690 ghost error message fails compile

I won't even try to explain the error reporting code, because
it would have no basis in reality. However this change appears
to fix the symptom reported.
Merge pull request #2807 from som-snytt/issue/partest-dir

SI-7729 Does Par-Test work?  Absolutely!
Merge pull request #2804 from adriaanm/rebase-2763

SI-7690 ghost error message fails compile [Rebase of #2763]