magit-insert-untracked-files: respect status.showUntrackedFiles

Showing all untracked files can be slow.  So respect the Git variable
`status.showUntrackedFiles' allowing users to make it faster at the
cost of not being able to stage every untracked file individually.

That variable defaults to "normal" which means that when directory
contains nothing but untracked files then only the directory is shown.

Before respecting this variable Magit showed each untracked files using
an initially collapsed tree.  Doing that would probably be the more
reasonable default, but I will delay the final decision until after the
following and just do what Git would do for now.

In the future we might use `magit-status.showUntrackedFiles' instead of
sharing `status.showUntrackedFiles' with Git.  Then we could also have
another default ("all") without anyone being able to say that doing so
was wrong.