We will need this because under some conditions we are going to use this
function to edit a range of commits, and we can't set merge commits to "edit".
This corresponds to the code in startInteractiveRebaseWithEdit which has similar
logic.
It is a bit unfortunate that we will have these two different ways of setting
todos to edit: startInteractiveRebaseWithEdit does it after stopping in the
rebase, in the Then function of its refresh, but InteractiveRebase does it in
the daemon with a ChangeTodoActionsInstruction. It still makes sense though,
given how InteractiveRebase works.
This not only affects "edit", but also "drop", "fixup", and "squash".
Previously, when trying to use these for a range selection that includes a merge
commit, they would fail with the cryptic error message "Some todos not found in
git-rebase-todo"; now they simply exclude the merge commit. I'm not sure if one
is better or worse than the other, and we should probably simply disable the
commands when a merge commit is selected, but that's out of scope in this PR.
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