RFR: JDK-8246484 Verify patch at start of COMPARE_BUILD=PATCH run

[Magnus Ihse Bursie]

I have supplied an incorrect patch to COMPARE_BUILD=PATCH one time too 
many. This patch adds an early verification that the patch can be 
applied, before starting the full comparison build.

Bug: https://bugs.openjdk.java.net/browse/JDK-8246484
WebRev: http://cr.openjdk.java.net/~ihse/JDK-8246484-verify-PATCH/webrev.01
/Magnus