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.

When the variable is unset `git status` defaults to `normal` while
`magit-insert-untracked-files` defaults to `all`, allowing them to
be staged individually.

Note that when `normal` is used the untracked files are still displayed
as a tree, but some of the trees cannot be expanded.  If directory
`mixed` contains an untracked and a tracked file then it is expandable
and expanding it will show the untracked file.  While the directory
`untracked` contains only untracked files and cannot be expanded.  With
everything expanded this would look like this:

Untracked files (2)
mixed/
mixed/untracked
untracked/

@Niluge-KiWi how does that look as a fix for #1599?