It used to work on the assumption that rebasing commits in lazygit's model
correspond one-to-one to lines in the git-rebase-todo file, which isn't
necessarily true (e.g. when users use "git rebase --edit-todo" at the custom
command prompt and add a "break" between lines).
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.
It's not so much the total number of commits that matters here, it's just
whether we are on the first one. (This includes the other condition.)
This allows us to get rid of the condition in rebase.go.
Instead of rebasing from the commit below the current one and then setting the
current one to "edit", we rebase from the current one and insert a "break" after
it. In most cases the behavior is exactly the same as before, except that the
new method also works if the current commit is a merge commit. This is useful if
you want to create a new commit at the very beginning of your branch (by editing
the last commit before your branch).
I noticed that `$GIT_SEQUENCE_EDITOR` is overridden in `PrepareInteractiveRebaseCommand`
but not in `runSkipEditorCommand`.
Before this change, some commands such as `SquashAllAboveFixupCommits`
would not work when a different sequence editor, e.g.
[git-interactive-rebase-tool](https://github.com/MitMaro/git-interactive-rebase-tool)
is configured.
