Fix problem with drag-select not clearning multiple cursors

Issue #2486

Author: marijnh

lib/codemirror.js

@@ -2644,6 +2644,7 @@
     if (!addNew) {
       ourIndex = 0;
       setSelection(doc, new Selection([ourRange], 0), sel_mouse);
+      startSel = doc.sel;
     } else if (ourIndex > -1) {
       replaceOneSelection(doc, ourIndex, ourRange, sel_mouse);
     } else {