Add missing support for the latest SPI status codes.

SPI_result_code_string() was missing support for SPI_OK_TD_REGISTER,
and in v15 and later, it was missing support for SPI_OK_MERGE, as was
pltcl_process_SPI_result().

The last of those would trigger an error if a MERGE was executed from
PL/Tcl. The others seem fairly innocuous, but worth fixing.

Back-patch to all supported branches. Before v15, this is just adding
SPI_OK_TD_REGISTER to SPI_result_code_string(), which is unlikely to
be seen by anyone, but seems worth doing for completeness.

Reviewed by Tom Lane.

Discussion:
  https://postgr.es/m/CAEZATCUg8V%2BK%2BGcafOPqymxk84Y_prXgfe64PDoopjLFH6Z0Aw%40mail.gmail.com
  https://postgr.es/m/CAEZATCUMe%2B_KedPMM9AxKqm%3DSZogSxjUcrMe%2BsakusZh3BFcQw%40mail.gmail.com

doc/src/sgml/spi.sgml

@@ -437,6 +437,15 @@ typedef struct SPITupleTable
      </listitem>
     </varlistentry>
 
+    <varlistentry>
+     <term><symbol>SPI_OK_MERGE</symbol></term>
+     <listitem>
+      <para>
+       if a <command>MERGE</command> was executed
+      </para>
+     </listitem>
+    </varlistentry>
+
     <varlistentry>
      <term><symbol>SPI_OK_INSERT_RETURNING</symbol></term>
      <listitem>

src/backend/executor/spi.c

@@ -2029,6 +2029,10 @@ SPI_result_code_string(int code)
 			return "SPI_OK_REL_REGISTER";
 		case SPI_OK_REL_UNREGISTER:
 			return "SPI_OK_REL_UNREGISTER";
+		case SPI_OK_TD_REGISTER:
+			return "SPI_OK_TD_REGISTER";
+		case SPI_OK_MERGE:
+			return "SPI_OK_MERGE";
 	}
 	/* Unrecognized code ... return something useful ... */
 	sprintf(buf, "Unrecognized SPI code %d", code);

src/pl/tcl/pltcl.c

@@ -2441,6 +2441,7 @@ pltcl_process_SPI_result(Tcl_Interp *interp,
 		case SPI_OK_INSERT:
 		case SPI_OK_DELETE:
 		case SPI_OK_UPDATE:
+		case SPI_OK_MERGE:
 			Tcl_SetObjResult(interp, Tcl_NewWideIntObj(ntuples));
 			break;