main.view: Avoid ambiguosly overloaded shortcut

Alt+D is the "Diffstat" shortcut but it was also being used to
trigger the "Diff" entry on the main menu.

Remove the main menu shortcut so that the "Diffstat" action
does not conflict with it.

Closes #159

Reported-by: Rustam Safin <rustiksnegovik@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>