Fix accidental 'let'

Author: marijnh

addon/merge/merge.js

@@ -286,7 +286,7 @@
     var vpEdit = dv.edit.getViewport(), vpOrig = dv.orig.getViewport();
     var outerTop = dv.mv.wrap.getBoundingClientRect().top
     var sTopEdit = outerTop - dv.edit.getScrollerElement().getBoundingClientRect().top + dv.edit.getScrollInfo().top
-    let sTopOrig = outerTop - dv.orig.getScrollerElement().getBoundingClientRect().top + dv.orig.getScrollInfo().top;
+    var sTopOrig = outerTop - dv.orig.getScrollerElement().getBoundingClientRect().top + dv.orig.getScrollInfo().top;
     for (var i = 0; i < dv.chunks.length; i++) {
       var ch = dv.chunks[i];
       if (ch.editFrom <= vpEdit.to && ch.editTo >= vpEdit.from &&