Add tab-completion for ALTER TABLE ALTER CONSTRAINT in psql.

Back-patch to 9.4 where ALTER TABLE ALTER CONSTRAINT was added.

Michael Paquier, bug reported by Andrey Lizenko.

src/bin/psql/tab-complete.c

@@ -1605,7 +1605,7 @@ psql_completion(const char *text, int start, int end)
 	else if (pg_strcasecmp(prev4_wd, "ALTER") == 0 &&
 			 pg_strcasecmp(prev3_wd, "TABLE") == 0 &&
 			 pg_strcasecmp(prev_wd, "ALTER") == 0)
-		COMPLETE_WITH_ATTR(prev2_wd, " UNION SELECT 'COLUMN'");
+		COMPLETE_WITH_ATTR(prev2_wd, " UNION SELECT 'COLUMN' UNION SELECT 'CONSTRAINT'");
 
 	/* ALTER TABLE xxx RENAME */
 	else if (pg_strcasecmp(prev4_wd, "ALTER") == 0 &&
@@ -1655,12 +1655,13 @@ psql_completion(const char *text, int start, int end)
 		COMPLETE_WITH_ATTR(prev3_wd, "");
 
 	/*
-	 * If we have ALTER TABLE <sth> DROP|RENAME|VALIDATE CONSTRAINT, provide
+	 * If we have ALTER TABLE <sth> ALTER|DROP|RENAME|VALIDATE CONSTRAINT,
-	 * list of constraints
+	 * provide list of constraints
 	 */
 	else if (pg_strcasecmp(prev5_wd, "ALTER") == 0 &&
 			 pg_strcasecmp(prev4_wd, "TABLE") == 0 &&
-			 (pg_strcasecmp(prev2_wd, "DROP") == 0 ||
+			 (pg_strcasecmp(prev2_wd, "ALTER") == 0 ||
+			  pg_strcasecmp(prev2_wd, "DROP") == 0 ||
 			  pg_strcasecmp(prev2_wd, "RENAME") == 0 ||
 			  pg_strcasecmp(prev2_wd, "VALIDATE") == 0) &&
 			 pg_strcasecmp(prev_wd, "CONSTRAINT") == 0)