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

[Magnus Ihse Bursie]

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

/Magnus