patch 8.0.0448: some macros are in lower case

Problem:    Some macros are in lower case, which can be confusing.
Solution:   Make a few lower case macros upper case.
diff --git a/src/diff.c b/src/diff.c
index 4352699..8c70e0c 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -2333,7 +2333,7 @@
 		    end_skip = 0;
 	    }
 
-	    buf_empty = bufempty();
+	    buf_empty = BUFEMPTY();
 	    added = 0;
 	    for (i = 0; i < count; ++i)
 	    {