Stop blocking selection-undo in read-only mode

Closes codemirror#4090

Author: marijnh

lib/codemirror.js

@@ -4419,7 +4419,7 @@
 
   // Revert a change stored in a document's history.
   function makeChangeFromHistory(doc, type, allowSelectionOnly) {
-    if (doc.cm && doc.cm.state.suppressEdits) return;
+    if (doc.cm && doc.cm.state.suppressEdits && !allowSelectionOnly) return;
 
     var hist = doc.history, event, selAfter = doc.sel;
     var source = type == "undo" ? hist.done : hist.undone, dest = type == "undo" ? hist.undone : hist.done;