mipsy_web: fix undo/redo

Calling setValue on the editor causes it to drop the current
undo/redo history, and set_editor_value() is called whenever
the user changes the source code. This resulted in undo/redo
not working at all. This commit changes set_editor_value to
only call setValue() when there's a difference between the
new and old value.

There's likely a nicer way of fixing this that involves
set_editor_value() not being called as often -- but this
change seemed least likely to introduce a bug related to
a desync between the editor's and the app's view of the
editor contents.

crates/mipsy_web/index.html

@@ -131,7 +131,11 @@
       }
 
       function set_editor_value(value="") {
-        window.editor.setValue(value);
+        // Don't unnecesarily call editor.setValue(),
+        // since setValue will also reset undo history.
+        if (get_editor_value() !== value) {
+          window.editor.setValue(value);
+        }
       }
 
       function get_editor_value() {