gitk: Allow user to choose whether to see the diff, old file, or new file

gitk: Allow user to choose whether to see the diff, old file, or new file

This adds a set of radiobuttons that select between displaying the full
diff (both - and + lines), the old file (suppressing the + lines) and the
new file (suppressing the - lines) in the diff display window.

Signed-off-by: Paul Mackerras <paulus@samba.org>