[feature request] git add completion should exclude staged content

Dear Git Hackers,

I have a feature request for `git add` auto completion:

`git add` auto completion suggests all files / directories, filtered by nothing. I guess it would be much nicer (as in increasing productivity) if it would only suggest unstaged content, as reported by `git status`, because that would be the only content one would be able to add.

Example:

    $ git status
    # On branch develop
    # Changes to be committed:
    #   (use "git reset HEAD <file>..." to unstage)
    #
    #       modified:   src/main/scala/XYChart.scala
    #
    # Untracked files:
    #   (use "git add <file>..." to include in what will be committed)
    #
    #       notes/0.2.0.markdown

    $ git add <auto-complete>
    build.sbt              .ensime                .git/                  project/               scalastyle-config.xml  todo
    COPYING                .ensime_lucene/        notes/                 README.md              src/
    $ git add

Where it should be:

    $ git add <auto-complete>
    $ git add notes/0.2.0.markdown

... because `notes/0.2.0.markdown` is the only thing I can add.