widgets/diff: simplify if logic

Pull the check for s.staged out of the subordinate if/elif statement
into the parent if statement.

Signed-off-by: Daniel Harding <dharding@living180.net>
widgets/diff: deleted files context menu

The Stage/Unstage/Revert actions for the Diff widget context menu don't
make any sense for deleted files, so don't show them in that case.

Signed-off-by: Daniel Harding <dharding@living180.net>
Merge pull request #396 from living180/deleted_context_menu

diff: improve context menu for deleted files

Signed-off-by: David Aguilar <davvid@gmail.com>