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.
