So far the algorithm worked on the assumption that the output of the "git show"
command corresponds one-to-one to the lines of the rebase-todo file. This
assumption doesn't hold once we start to include todo lines that don't have a
sha (like update-ref), or when the todo file contains multiple entries for the
same sha. This should never happen normally, but it can if users manually edit
the todo file and duplicate a line.
a0d179b6dc