textLinesMutator: fix insertions with newlines at the end of a line. In

such cases the remaining part of the old line is directly pushed to the
splice and we need to ensure it is not an empty string.

src/static/js/Changeset.js

@@ -915,8 +915,9 @@ exports.textLinesMutator = (lines) => {
           curLine += newLines.length;
           // insert the remaining chars from the "old" line (e.g. the line we were in
           // when we started to insert new lines)
-          curSplice.push(theLine.substring(lineCol));
-          curCol = 0; // TODO(doc) why is this not set to the length of last line?
+          const remaining = theLine.substring(lineCol);
+          if (remaining !== '') curSplice.push(remaining);
+          curCol = 0;
         } else {
           Array.prototype.push.apply(curSplice, newLines);
           curLine += newLines.length;