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>
widgets/status: fix unstaged deleted diff

2cbd38e2edca6ae6aa732b7afb809fb4b2a7dbf0 broke displaying the diff for
an unstaged deleted file.  In the call to cmds.do(cmds.Diff, ...),
item.deleted was being passed as a positional argument, which meant it
was actually being assigned to the cached argument, rather than the
deleted argument, resulting in an empty diff.  Change to pass as a
keyword argument to avoid this.

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>
Merge pull request #395 from living180/fix_2cbd38e2

status: fix diff for unstaged deleted files

Signed-off-by: David Aguilar <davvid@gmail.com>
urls: use https:// urls everywhere

Signed-off-by: David Aguilar <davvid@gmail.com>