don't default to --ln-untracked when $new=='--'

This was made to avoid having to checkout the worktree, but since
fe95cc4 (take --whole-tree into account when diffing with worktree,
2016-05-14), we do copy the worktree anyway, so this is not needed
anymore.

git-latexdiff

@@ -376,9 +376,6 @@ done
 if test -z "$new" ; then
     new=HEAD
 fi
-if test "$new" = "--"; then
-    ln_untracked=1
-fi
 
 if test -z "$old" ; then
     echo "fatal: Please, provide at least one revision to diff with."