Fix incorrect PITR message for transaction ROLLBACK PREPARED

Reaching PITR on such a transaction would cause the generation of a LOG
message mentioning a transaction committed, not aborted.

Oversight in 4f1b890.

Author: Simon Riggs
Discussion: https://postgr.es/m/CANbhV-GJ6KijeCgdOrxqMCQ+C8QiK657EMhCy4csjrPcEUFv_Q@mail.gmail.com
Backpatch-through: 9.6

Author: michaelpq

src/backend/access/transam/xlog.c

@@ -5838,7 +5838,7 @@ recoveryStopsBefore(XLogReaderState *record)
 		xl_xact_abort *xlrec = (xl_xact_abort *) XLogRecGetData(record);
 		xl_xact_parsed_abort parsed;
 
-		isCommit = true;
+		isCommit = false;
 		ParseAbortRecord(XLogRecGetInfo(record),
 						 xlrec,
 						 &parsed);