Re-enable some upstream code that was disabled earlier, in the 8.4 merge.

src/backend/access/transam/xlog.c

@@ -829,13 +829,9 @@ XLogInsert_Internal(RmgrId rmid, uint8 info, XLogRecData *rdata, TransactionId h
 		}
  	}
 
-	/* GPDB_84_MERGE_FIXME: This cross-check was added in upstream, but it's failing
-	 * in Startup pass 2. Disable it for now. */
-#if 0
 	/* cross-check on whether we should be here or not */
 	if (!XLogInsertAllowed())
 		elog(ERROR, "cannot make new WAL entries during recovery");
-#endif
 
 	/* info's high bits are reserved for use by me */
 	if (info & XLR_INFO_MASK)
@@ -8072,12 +8068,7 @@ InitXLOGAccess(void)
 {
 	/* ThisTimeLineID doesn't change so we need no lock to copy it */
 	ThisTimeLineID = XLogCtl->ThisTimeLineID;
-	/* GPDB_84_MERGE_FIXME: Disabled, because FTS process was tripping it.
-	 * This assertion was added by the merge, so I suspect it's been wrong
-	 * all along, but we haven't noticed. */
-#if 0
 	Assert(ThisTimeLineID != 0);
-#endif
 
 	/* Use GetRedoRecPtr to copy the RedoRecPtr safely */
 	(void) GetRedoRecPtr();