vim-patch:8.0.0448: some macros are lower case (#7936)

Problem:    Some macros are in lower case, which can be confusing.
Solution:   Make a few lower case macros upper case.
b5aedf3e22

ref #6297

src/nvim/diff.c

@@ -2267,7 +2267,7 @@ void ex_diffgetput(exarg_T *eap)
         }
       }
 
-      buf_empty = bufempty();
+      buf_empty = BUFEMPTY();
       added = 0;
 
       for (i = 0; i < count; ++i) {