Instead, derive it from context at display time (if we're rebasing, it's the
first non-todo commit). This fixes the problem that unfolding the current
commit's files in the local commits panel would show junk in the frame's title.
Along the way we make sure to only display the "<--- YOU ARE HERE" string in the
local commits panel; previously it would show for the top commit of a branch or
tag if mid-rebase.
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
