When pulling/pushing/fast-forwarding a branch, show this state in the branches
list for that branch for as long as the operation takes, to make it easier to
see when it's done (without having to stare at the status bar in the lower
left).
This will hopefully help with making these operations feel more predictable, now
that we no longer show a loader panel for them.
test: add an integration test for checkout branch by name
fix: fix full ref name of detached head
refactor: refactor current branch loader
chore: use field name explicitly
