quote: use isalnum() to check for alphanumeric characters

isalnum(c) is equivalent to isalpha(c) || isdigit(c), so use the
former instead.  The result is shorter, simpler and slightly more
efficient.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

quote.c

@@ -55,7 +55,7 @@ void sq_quote_buf_pretty(struct strbuf *dst, const char *src)
 	}
 
 	for (p = src; *p; p++) {
-		if (!isalpha(*p) && !isdigit(*p) && !strchr(ok_punct, *p)) {
+		if (!isalnum(*p) && !strchr(ok_punct, *p)) {
 			sq_quote_buf(dst, src);
 			return;
 		}