magit-wash-diff: properly cancel empty section

In `magit-wash-diff' properly cancel the section if it is empty by
setting SECTION to nil.  This still does not insert the "(empty)"
string, because that is implemented in `magit-cmd-insert-section',
not `magit-with-section'.  I am still undecided but I will probably
not move that feature and instead refactor diff washing so that it
is done in only one wash cycle (like most other things).
magit-insert-untracked-files: make consistent and shorter

Use `git status' instead of `git ls-files' to get list of untracked
files, as recommended in the latter's manpage.  Reduce code size by
using `magit-with-section' instead of `magit-git-insert-section'.
The latter should only be used for complex washing.  (But in many
cases that isn't so yet.)

Always rely on `status.showUntrackedFiles'.  Previously that was only
used when its value was "no" and the "normal" (show untracked files
and directories) and "all" (also show individual files in untracked
directories) had to be set using `magit-status-verbose-untracked'.
That is highly confusing, either respect the git variable or don't.