[feature request] Make "commit --only" work with new files

Hello,

currently "git commit --only <file>" only works if <file> is already
checked into the repo, but not with newly created and still untracked
files (builtin/commit.c:list_path() throws the error "error: pathspec
'<file>' did not match any file(s) known to git.")

I don't think this limitation is intented. I've had a look at the
relevant part of builtin/commit.c, but unfortunately it wasn't obvious
to me how to fix this.

Regards,
Luis Ressel