tab_complete for copy(merge

hi.

in v17, we support COPY(MERGE RETURNING)
we can add tab_complete for it.

    /* Complete COPY ( with legal query commands */
    else if (Matches("COPY|\\copy", "("))
-               COMPLETE_WITH("SELECT", "TABLE", "VALUES", "INSERT
INTO", "UPDATE", "DELETE FROM", "WITH");
+               COMPLETE_WITH("SELECT", "TABLE", "VALUES", "INSERT
INTO", "UPDATE", "DELETE FROM", "WITH", "MERGE INTO");