In bisect-runner.pl, fall back to context diffs for ancient patch binaries.

In bisect-runner.pl, fall back to context diffs for ancient patch binaries.

AIX supplies a pre-historic patch program, which certainly predates Linux
and is probably older than NT. It can't cope with unified diffs. Meanwhile,
it's hard enough to get git diff to output context diffs, let alone git show,
and nearly all the patches embedded here are unified. So it seems that the
path of least resistance is to convert unified diffs to context diffs.