This is useful if you want to move a range of commits, so you select them, and
then realize it's better to do it in an interactive rebase. Pressing 'i'
preserves the range now.
We want to show an error when the user tries to invoke an action that expects only
a single item to be selected.
We're using the GetDisabledReason field to enforce this (as well as DisabledReason
on menu items).
I've created a ListControllerTrait to store some shared convenience functions for this.
The only time we should call SetSelectedLineIdx is when we are happy for a
select range to be retained which means things like moving the selected line
index to top top/bottom or up/down a page as the user navigates.
But in every other case we should now call SetSelection because that will
set the selected index and cancel the range which is almost always what we
want.
This adds range select ability in two ways:
1) Sticky: like what we already have with the staging view i.e. press v then use arrow keys
2) Non-sticky: where you just use shift+up/down to expand the range
The state machine works like this:
(no range, press 'v') -> sticky range
(no range, press arrow) -> no range
(no range, press shift+arrow) -> nonsticky range
(sticky range, press 'v') -> no range
(sticky range, press arrow) -> sticky range
(sticky range, press shift+arrow) -> nonsticky range
(nonsticky range, press 'v') -> no range
(nonsticky range, press arrow) -> no range
(nonsticky range, press shift+arrow) -> nonsticky range
