patch 8.2.2563: cannot use multibyte characters for folding in 'fillchars'

Problem:    Cannot use multibyte characters for folding in 'fillchars'.
Solution:   Port pull request 11568 to Vim. (Yegappan Lakshmanan,
            closes #7924)
diff --git a/src/macros.h b/src/macros.h
index 3db2456..5a57a89 100644
--- a/src/macros.h
+++ b/src/macros.h
@@ -388,3 +388,10 @@
 
 // Inlined version of ga_grow().  Especially useful if "n" is a constant.
 #define GA_GROW(gap, n) (((gap)->ga_maxlen - (gap)->ga_len < n) ? ga_grow_inner((gap), (n)) : OK)
+
+#ifndef MIN
+# define MIN(a, b) ((a) < (b) ? (a) : (b))
+#endif
+#ifndef MAX
+# define MAX(a, b) ((a) > (b) ? (a) : (b))
+#endif