This is useful when working with stacked branches, because you can now move
"pick" entries across an update-ref command and you can tell exactly which
branch the commit will end up in.
It's also useful to spot situations where the --update-refs option didn't work
as desired. For example, if you duplicate a branch and want to rebase only one
of the branches but not the other (maybe for testing); if you have
rebase.updateRefs=true in your git config, then rebasing one branch will move
the other branch along. To solve this we'll have to introduce a way to delete
the update-ref entry (maybe by hitting backspace?); this is out of scope for
this PR, so for now users will have to type "git rebase --edit-todo" into the
custom command prompt to sort this out.
We will also have to prevent users from trying to turn update-ref commands into
other commands like "pick" or "drop"; we'll do this later in this branch.
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