do not save everything in command history of em/wasm demo

share/emscripten/gpac.html

@@ -664,7 +664,7 @@ <h4>Performances</h4>
         s = FS.analyzePath('/idbfs/.gpac/history.txt', true);
         if (s.error) return;
         let strs = FS.readFile('/idbfs/.gpac/history.txt', { encoding: "utf8"}).split('\n');
-        strs.forEach(s => { if (s.length) cmd_history.push(s); });
+        strs.forEach(s => { if (s.length && (cmd_history.indexOf(s)<0)) cmd_history.push(s); });
       }
       //save command history to /idbfs/.gpac/history.txt
       function save_history() {
@@ -791,6 +791,10 @@ <h4>Performances</h4>
        }
 
        //save history
+       let cmd_idx = cmd_history.indexOf(cmd);
+       if (cmd_idx>=0) {
+          cmd_history.splice(cmd_idx, 1);
+       }
        cmd_history.push(cmd);
        save_history();
        cmd_idx = -1;
@@ -1218,6 +1222,10 @@ <h4>Performances</h4>
             }
             return;
           }
+          if (args[0]=='clear') {
+            if (outputElement) outputElement.innerText = '';
+            return;
+          }
 
 
           if (args[0]=='p') {