Merge pull request #7046 from quantopian/fix-history-undo-load

BUG: Prevent users from undoing the initial document load with CTRL-Z.

IPython/html/static/edit/js/editor.js

@@ -36,6 +36,10 b' function($,'
             .then(function(model) {
                 cm.setValue(model.content);
                 
+                // Setting the file's initial value creates a history entry,
+                // which we don't want.
+                cm.clearHistory();
+
                 // Find and load the highlighting mode
                 var modeinfo = CodeMirror.findModeByMIME(model.mimetype);
                 if (modeinfo) {