views.status: Improve selection handling across updates

views.status: Improve selection handling across updates

We now manage the selection across updates much more carefully.
When items are staged they are selected and their
diff is automatically shown.  Vice-versa for unstaged.

A group of selected items retains their selection across
category moves.  If an item moves into a new category,
we scroll the viewport to the item in that category.

Priority is given to the current, partially-staged diff
when interactively staging hunks.  Otherwise, we would
be breaking context by showing the unstaged diff and then
switching to the staged fragment.

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