fix for #1678, undo no longer clears cells

I found another bug where switching the cell type causes the loss of all
undo history for that cell. With this commit, switching the cell type
simply resets the history

IPython/frontend/html/notebook/static/js/codecell.js

@@ -267,6 +267,9 var IPython = (function (IPython) {
         if (data.cell_type === 'code') {
             if (data.input !== undefined) {
                 this.set_text(data.input);
+                // make this value the starting point, so that we can only undo
+                // to this state, instead of a blank cell
+                this.code_mirror.clearHistory();
             }
             if (data.prompt_number !== undefined) {
                 this.set_input_prompt(data.prompt_number);

IPython/frontend/html/notebook/static/js/notebook.js

@@ -602,6 +602,9 var IPython = (function (IPython) {
                     text = '';
                 }
                 target_cell.set_text(text);
+                // make this value the starting point, so that we can only undo
+                // to this state, instead of a blank cell
+                target_cell.code_mirror.clearHistory();
                 source_element.remove();
                 this.dirty = true;
             };
@@ -623,6 +626,9 var IPython = (function (IPython) {
                 // The edit must come before the set_text.
                 target_cell.edit();
                 target_cell.set_text(text);
+                // make this value the starting point, so that we can only undo
+                // to this state, instead of a blank cell
+                target_cell.code_mirror.clearHistory();
                 source_element.remove();
                 this.dirty = true;
             };
@@ -645,6 +651,9 var IPython = (function (IPython) {
                 // The edit must come before the set_text.
                 target_cell.edit();
                 target_cell.set_text(text);
+                // make this value the starting point, so that we can only undo
+                // to this state, instead of a blank cell
+                target_cell.code_mirror.clearHistory();
                 source_element.remove();
                 this.dirty = true;
             };
@@ -667,6 +676,9 var IPython = (function (IPython) {
                 // The edit must come before the set_text.
                 target_cell.edit();
                 target_cell.set_text(text);
+                // make this value the starting point, so that we can only undo
+                // to this state, instead of a blank cell
+                target_cell.code_mirror.clearHistory();
                 source_element.remove();
                 this.dirty = true;
             };
@@ -693,6 +705,9 var IPython = (function (IPython) {
                 target_cell.set_level(level);
                 target_cell.edit();
                 target_cell.set_text(text);
+                // make this value the starting point, so that we can only undo
+                // to this state, instead of a blank cell
+                target_cell.code_mirror.clearHistory();
                 source_element.remove();
                 this.dirty = true;
             };

IPython/frontend/html/notebook/static/js/textcell.js

@@ -158,6 +158,9 var IPython = (function (IPython) {
         if (data.cell_type === this.cell_type) {
             if (data.source !== undefined) {
                 this.set_text(data.source);
+                // make this value the starting point, so that we can only undo
+                // to this state, instead of a blank cell
+                this.code_mirror.clearHistory();
                 this.set_rendered(data.rendered || '');
                 this.rendered = false;
                 this.render();