git commit --only -- $path when $path already has staged content

Hello.

The manpage of git commit reads:

  --only

      Make a commit only from the paths specified on the command line,
      disregarding any contents that have been staged so far. This is
      the default mode of operation of git commit if any paths are given
      on the command line, in which case this option can be omitted. [...]

But that seems to be only true for other content (i.e. other files not specified in the command line).

If I do:

    # some repo with a file in it
    git init
    echo content >foo && git add foo && git commit -m foo

    # modify and stage a file
    echo other >> foo && git add foo

    # modify the same file even further but don't stage
    echo bar >> foo

    # commit with path specified on command line with explicit '--only'
    git commit --only -m'.' -- foo

    # but everything was commited
    git status -s
    <empty>

I would expect to only get the unstaged changes in the commit. Could anyone shed some light, please?

Thanks,
  Stefan