The only exception is when moving a custom patch for an entire commit to an
earlier commit; in this case the source commit becomes empty, but we want to
keep it, mainly for consistency with moving the patch to a later commit, which
behaves the same.
In all other cases where we rebase, it's confusing when empty commits are kept;
the most common example is rebasing a branch onto master, where master already
contains some of the commits of our branch. In this case we simply want to drop
these.
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.
