The main reason for doing this (besides the reasons given for Status in the
previous commit) is that it allows us to easily convert from TodoCommand to
Action and back. This will be needed later in the branch. Fortunately,
TodoCommand is one-based, so this allows us to add an ActionNone constant with
the value 0.
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.
