We're not fully standardising here: different contexts can store their range state however
they like. What we are standardising on is that now the view is always responsible for
highlighting the selected lines, meaning the context/controller needs to tell the view
where the range start is.
Two convenient benefits from this change:
1) we no longer need bespoke code in integration tests for asserting on selected lines because
we can just ask the view
2) line selection in staging/patch-building/merge-conflicts views now look the same as in
list views i.e. the highlight applies to the whole line (including trailing space)
I also noticed a bug with merge conflicts not rendering the selection on focus though I suspect
it wasn't a bug with any real consequences when the view wasn't displaying the selection.
I'm going to scrap the selectedRangeBgColor config and just let it use the single line
background color. Hopefully nobody cares, but there's really no need for an extra config.
When we use the one panel for the entire commit message, its tricky to have a keybinding both for adding a newline and submitting.
By having two panels: one for the summary line and one for the description, we allow for 'enter' to submit the message when done from the summary panel,
and 'enter' to add a newline when done from the description panel. Alt-enter, for those who can use that key combo, also works for submitting the message
from the description panel. For those who can't use that key combo, and don't want to remap the keybinding, they can hit tab to go back to the summary panel
and then 'enter' to submit the message.
We have some awkwardness in that both contexts (i.e. panels) need to appear and disappear in tandem and we don't have a great way of handling that concept,
so we just push both contexts one after the other, and likewise remove both contexts when we escape.
