#60 Show diff on submit

[Johan Walles]

Change a bunch of files.
Submit.
Behold the submit dialog.

Current behaviour:
It has a text field for describing the change and a
list of changed files that I can (un)check depending on
which ones I want to submit.

Expected behaviour:
In addition to the two current panes, I'd like to have
a diff pane as well, where I can see what has changed
in the files. That would be a good help in deciding
what files I really want to submit.

I'm using 0.6.24.