RFR(XS): 8145828: JPRT hotspot push jobs should allow merge on push

[Mikael Vidstedt]

Please review this small change which relaxes the check made in JPRT at 
the time when a hotspot push job has finished successfully, and the 
changes are about to be pushed.

Currently the job will be marked as failed if a merge is needed, even if 
an automatic merge would complete successfully. This "safety mechanism" 
was reasonable when all pushes were made using JPRT, but since we 
started allowing direct pushes the failure rate has gone up. The 
assumption here is that the code changes will very rarely overlap, and 
even more rarely conflict, so if the automatic merge is successful then 
it's highly likely that the resulting code would (also) pass the JPRT 
testing.

This change enables the automatic merge to be attempted, and if it is 
successful the resulting changes will be pushed.

Bug: https://bugs.openjdk.java.net/browse/JDK-8145828
Webrev: http://cr.openjdk.java.net/~mikael/webrevs/8145828/webrev.00/webrev/

Cheers,
Mikael