Detect ordering-only differences in text files

This bug was originally reported by Paul Wise (@pabs) in Debian bug #1022145:

Package: diffoscope
Version: 224
Severity: wishlist

It would be nice if diffoscope could detect that text files are only
different because of ordering differences, currently it cannot:

   $ (echo foo ; echo bar) > foo
   $ (echo bar ; echo foo) > bar
  =20
   $ diffoscope foo bar
   --- foo
   +++ bar
   @@ -1,2 +1,2 @@
   -foo
    bar
   +foo
  =20
   $ echo $?
   1
  =20
   $ (echo foo ; echo bar ; echo baz ; echo biz ; echo buz) > foo
   $ (echo bar ; echo foo ; echo baz ; echo biz ; echo buz) > bar
  =20
   $ diffoscope foo bar
   --- foo
   +++ bar
   @@ -1,5 +1,5 @@
   -foo
    bar
   +foo
    baz
    biz
    buz
  =20
   $ echo $?
   1