fix computation of chunk lines in current_line() so that chunk header and footer are not included

- if chunk has length 1, returned vector has only one instead of 2 elements
- if (and only if) chunk has length 0, line with chunk header is reported

Tested with the following file:

    Test.

    ```{r echo=F}
    options(knitr.package.verbose=T)
    opts_chunk$set()
    ```

    Bar.

    ```{r}
    knitr:::current_lines(4)
    knitr:::current_lines(5)
    knitr:::current_lines(6)
    ```
    ```{r}
    3
    ```

    Baz.
Merge pull request #692 from krlmlr/lines-one-off

fix computation of chunk lines in current_line() so that chunk header and footer are not included