have shift-m merge below if one cell selected

notebook/static/notebook/js/actions.js

@@ -380,13 +380,25 @@ define(function(require){
                 env.notebook.merge_cell_below();
             }
         },
-        'merge-cells' : {
+        'merge-selected-cells' : {
             help : 'merge selected cells',
             help_index: 'el',
             handler: function(env) {
                 env.notebook.merge_selected_cells();
             }
         },
+        'merge-cells' : {
+            help : 'merge selected cells, or current cell with cell below if only one cell selected',
+            help_index: 'el',
+            handler: function(env) {
+                var l = env.notebook.get_selected_cells_indices().length
+                if(l == 1){
+                    env.notebook.merge_cell_below();
+                } else {
+                    env.notebook.merge_selected_cells();
+                }
+            }
+        },
         'show-command-palette': {
             help_index : 'aa',
             help: 'open the command palette',