patch 8.1.2279: computation of highlight attributes is too complicated

Problem:    Computation of highlight attributes is too complicated.
Solution:   Simplify the attribute computation and make it more consistent.
            (closes #5190)  Fix that 'combine' set to zero doesn't work.

src/version.c

@@ -741,6 +741,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2279,
 /**/
     2278,
 /**/