Fixed side-by-side diff bugs related to binary diff and more.

The fixed bugs:

 * "Binary files differ" did not show up either in unidiff or
   side-by-side-diff.
 * Subproject diffs did not work for side-by-side diffs.
 * The ssdiff link on diff pages did not conserve the path.

Signed-off-by: Ragnar Ouchterlony <ragnar@lysator.liu.se>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>