save explicitly specified files even if they are ignored by git

From tarsius's pull request

magit-wip.el (part of https://github.com/magit/magit) did not check
whether git ignores the file being saved before calling git wip save
.... I created a patch (magit/magit#614) to fix that but it hasn't
been merged yet. Also I consider adding an option that would allow
users to choose whether such files should be tracked in the wip branch
or not.

Currently git-wip does not support adding a specific ignored file to
the wip branch. The --ignored option only works when no file is
specified explicitly. I think it makes sense to always add explicitly
specified but ignored files without requiring the --ignored to be
specified. In my eyes the file being specified explicitly should be
enough reason to believe the caller actually wants it added.
Always use "$@" for file name

From vanicat's pull request

The posix shell do not interpret local files = $@ as a way to set file as
an array. So when there is more than one files, or whin the file name contain a space, this will fail

It simpler to always use "$@" to manipulate the rest of the
command line, and it will correctly handle file name with space.