Previously we would render the diff for a directory to the main/secondary pair,
but a diff for a file to the staging/stagingSecondary pair. (And similar for
commit files: main/secondary for directories, but
patchBuilding/patchBuildingSecondary for files.)
I always found this confusing and couldn't really understand why we are doing
this; but now it gets in my way because I want to attach a controller to
main/secondary so that they can be focused. So change it to always use the main
context pair for everything we render from a side panel.