extra characters in md jupyter fenced code block octave output

[DrXyzzy]

1. Open a new .md file.
2. Enter the following in the Markdown Code(left) frame, including the triple backticks:

```octave
 x = 71;
 x
 ```

3. Run the cell by clicking "Run" in the Text (right) frame.
4. Observe the output is 714h.
5. The number of extraneous characters will change with the length of the variable name and the length of the value assigned.

[haraldschilly]

looks like Calysto/octave_kernel#221

[haraldschilly]

see #7227 (comment)