Improve line-jumping on show old / new source.

diffscuss-mode now appropriately figures out which line to jump to and
goes there, for both old and new source, in the cases where:

* The cursor is on a line starting with '-' (an old line)

* The cursor is on a line starting with '+' (a new line)

* The cursor is on a line starting with ' ' (a common line)

* The cursor is on a line starting with @@ (a hunk line)

Still need to deal with cases where the cursor is above a hunk line
(in the metadata) and maybe where the cursor is on a '\ No newline'
line.