Change timeline field of IDENTIFY_SYSTEM to int8

It was int4, but in the other replication commands, timelines are
returned as int8.

Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/7e4fdbdc-699c-4cd0-115d-fb78a957fc22@enterprisedb.com

doc/src/sgml/protocol.sgml

@@ -1838,7 +1838,7 @@ psql "dbname=postgres replication=database" -c "IDENTIFY_SYSTEM;"
        </varlistentry>
 
        <varlistentry>
-        <term><literal>timeline</literal> (<type>int4</type>)</term>
+        <term><literal>timeline</literal> (<type>int8</type>)</term>
         <listitem>
          <para>
           Current timeline ID. Also useful to check that the standby is

src/backend/replication/walsender.c

@@ -444,7 +444,7 @@ IdentifySystem(void)
 	TupleDescInitBuiltinEntry(tupdesc, (AttrNumber) 1, "systemid",
 							  TEXTOID, -1, 0);
 	TupleDescInitBuiltinEntry(tupdesc, (AttrNumber) 2, "timeline",
-							  INT4OID, -1, 0);
+							  INT8OID, -1, 0);
 	TupleDescInitBuiltinEntry(tupdesc, (AttrNumber) 3, "xlogpos",
 							  TEXTOID, -1, 0);
 	TupleDescInitBuiltinEntry(tupdesc, (AttrNumber) 4, "dbname",