Fix for change timeline field of IDENTIFY_SYSTEM to int8

Amendment to ec40f3422412cfdc140b5d3f67db7fd2dac0f1e2: We also need to
change the way the datum is supplied to int8.  Otherwise, the value is
still cut off as an int4, and it will crash on 32-bit platforms.

src/backend/replication/walsender.c

@@ -457,7 +457,7 @@ IdentifySystem(void)
 	values[0] = CStringGetTextDatum(sysid);
 
 	/* column 2: timeline */
-	values[1] = Int32GetDatum(currTLI);
+	values[1] = Int64GetDatum(currTLI);
 
 	/* column 3: wal location */
 	values[2] = CStringGetTextDatum(xloc);