Line options for git rebase --interactive

by Fabian for Git

On their ideas page for possible Google Summer of Code projects, the Git community proposes the addition of option arguments to the commands available on git-rebase-interactive todo lists, such as `squash --signoff f1a5c00`, which adds a ``Signed-off-by'' line to the commit message. During GSoC, I would like to refactor the git-rebase-interactive shell script such that it admits todo list commands with argument lists of varying length instead of solely nullary commands.