cache.h: hex2chr() - avoid -Wsign-compare warnings

Signed-off-by: NRamsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

cache.h

@@ -1264,8 +1264,8 @@ static inline unsigned int hexval(unsigned char c)
  */
 static inline int hex2chr(const char *s)
 {
-	int val = hexval(s[0]);
-	return (val < 0) ? val : (val << 4) | hexval(s[1]);
+	unsigned int val = hexval(s[0]);
+	return (val & ~0xf) ? val : (val << 4) | hexval(s[1]);
 }
 
 /* Convert to/from hex/sha1 representation */