cleanup: add isascii()

Add a standard definition of isascii() and use it to replace an open
coded high-bit test in pretty.c.  While we're there, write the ESC
char as the more commonly used '\033' instead of as 0x1b to enhance
its grepability.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pretty.c

@@ -75,8 +75,7 @@ static int get_one_line(const char *msg)
 /* High bit set, or ISO-2022-INT */
 int non_ascii(int ch)
 {
-	ch = (ch & 0xff);
-	return ((ch & 0x80) || (ch == 0x1b));
+	return !isascii(ch) || ch == '\033';
 }
 
 static int is_rfc2047_special(char ch)