patch 8.0.0736: OptionSet not triggered when entering diff mode

Problem:    The OptionSet autocommand event is not triggered when entering
            diff mode.
Solution:   use set_option_value() instead of setting the option directly.
            Change the tests from old to new style. (Christian Brabandt)

src/version.c

@@ -769,6 +769,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    736,
 /**/
     735,
 /**/