Initialize tsId and dbId fields in WAL record of COMMIT PREPARED.

Commit dd428c79 added dbId and tsId to the xl_xact_commit struct but missed
that prepared transaction commits reuse that struct. Fix that.

Because those fields were left unitialized, replaying a commit prepared WAL
record in a hot standby node would fail to remove the relcache init file.
That can lead to "could not open file" errors on the standby. Relcache init
file only needs to be removed when a system table/index is rewritten in the
transaction using two phase commit, so that should be rare in practice. In
HEAD, the incorrect dbId/tsId values are also used for filtering in logical
replication code, causing the transaction to always be filtered out.

Analysis and fix by Andres Freund. Backpatch to 9.0 where hot standby was
introduced.

src/backend/access/transam/twophase.c

@@ -2082,9 +2082,13 @@ RecordTransactionCommitPrepared(TransactionId xid,
 
 	/* Emit the XLOG commit record */
 	xlrec.xid = xid;
-	xlrec.crec.xact_time = GetCurrentTimestamp();
+
 	xlrec.crec.xinfo = initfileinval ? XACT_COMPLETION_UPDATE_RELCACHE_FILE : 0;
-	xlrec.crec.nmsgs = 0;
+
+	xlrec.crec.dbId = MyDatabaseId;
+	xlrec.crec.tsId = MyDatabaseTableSpace;
+
+	xlrec.crec.xact_time = GetCurrentTimestamp();
 	xlrec.crec.nrels = nrels;
 	xlrec.crec.nsubxacts = nchildren;
 	xlrec.crec.nmsgs = ninvalmsgs;