RFR: JDK-8074055 Improvements in compare.sh from build-infra

[Magnus Ihse Bursie]

This is a few improvements of the compare.sh script, originally targeted 
for JDK-8069064. It was forced to be left out at the last moment to 
avoid costly merge conflicts. :-(

Since then JDK-8073965 has modified compare.sh in somewhat the same way, 
but not as complete. Inspired by the changes in JDK-8073965, I improved 
it even further.

Bug: https://bugs.openjdk.java.net/browse/JDK-8074055
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8074055-compare-from-build-infra/webrev.01

/Magnus