patch 7.4.2019

Problem:    When ignoring case utf_fold() may consume a lot of time.
Solution:   Optimize for ASCII.

src/mbyte.c

@@ -3067,6 +3067,9 @@ utf_convert(
     int
 utf_fold(int a)
 {
+    if (a < 0x80)
+	/* be fast for ASCII */
+	return a >= 0x41 && a <= 0x5a ? a + 32 : a;
     return utf_convert(a, foldCase, (int)sizeof(foldCase));
 }