Versão final, com a adição de --changed-group-format
diff old-file new-file --old-group-format='<<<<<<< .old
%<=======
' --new-group-format='=======
%>>>>>>>> .new
' --changed-group-format='<<<<<<< .old
%<=======
%>>>>>>>> .new
'
que leva a
Top piece of bread
Mayonnaise
Lettuce
Tomato
Provolone
<<<<<< .old
Salami
Mortadella
Prosciutto
=====
Sauerkraut
Grilled Chicken
>>>>>>> .new
Creole Mustard
Bottom piece of bread