RFR: JDK-8141439 Fix compare.sh -o <otherdir> (broken by JDK-8136813)

[Tim Bell]

Hello Magnus:

> It turned out that the fix in JDK-8136813 was only tested with -2dirs 
> on compare.sh. When using -o, the compare script now fails to 
> correctly determine it's own location. This patch remedies that.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8141439
> WebRev: 
> http://cr.openjdk.java.net/~ihse/JDK-8141439-fix-compare-THIS/webrev.01

Looks good to me.

/Tim