By the way, you may woner why I'm calling diff with the order of arguments swapped to generate a reverse diff.

This is required because I'm ignoring the blanks so and a line is considered unchanged diff will show the version in the first file. So, for example, if the original file says

bar
[download]

and the new file says

if foo
        bar
[download]

this gives the right output

if foo
        bar
[download]

(with only the first line bold), instead of the wrong output

if foo
bar
[download]

(with only the first line bold again).