get_sha1_hex() micro-optimization

The function appeared high on a gprof output for a rev-list run of
a non-trivial size, and it was an obvious low-hanging fruit.

The code is from Linus.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

sha1_file.c

@@ -26,15 +26,43 @@ const unsigned char null_sha1[20];
 
 static unsigned int sha1_file_open_flag = O_NOATIME;
 
-static unsigned hexval(char c)
-{
-	if (c >= '0' && c <= '9')
-		return c - '0';
-	if (c >= 'a' && c <= 'f')
-		return c - 'a' + 10;
-	if (c >= 'A' && c <= 'F')
-		return c - 'A' + 10;
-	return ~0;
+static inline unsigned int hexval(unsigned int c)
+{
+	static signed char val[256] = {
+		 -1, -1, -1, -1, -1, -1, -1, -1,		/* 00-07 */
+		 -1, -1, -1, -1, -1, -1, -1, -1,		/* 08-0f */
+		 -1, -1, -1, -1, -1, -1, -1, -1,		/* 10-17 */
+		 -1, -1, -1, -1, -1, -1, -1, -1,		/* 18-1f */
+		 -1, -1, -1, -1, -1, -1, -1, -1,		/* 20-27 */
+		 -1, -1, -1, -1, -1, -1, -1, -1,		/* 28-2f */
+		  0,  1,  2,  3,  4,  5,  6,  7,		/* 30-37 */
+		  8,  9, -1, -1, -1, -1, -1, -1,		/* 38-3f */
+		 -1, 10, 11, 12, 13, 14, 15, -1,		/* 40-47 */
+		 -1, -1, -1, -1, -1, -1, -1, -1,		/* 48-4f */
+		 -1, -1, -1, -1, -1, -1, -1, -1,		/* 50-57 */
+		 -1, -1, -1, -1, -1, -1, -1, -1,		/* 58-5f */
+		 -1, 10, 11, 12, 13, 14, 15, -1,		/* 60-67 */
+		 -1, -1, -1, -1, -1, -1, -1, -1,		/* 68-67 */
+		 -1, -1, -1, -1, -1, -1, -1, -1,		/* 70-77 */
+		 -1, -1, -1, -1, -1, -1, -1, -1,		/* 78-7f */
+		 -1, -1, -1, -1, -1, -1, -1, -1,		/* 80-87 */
+		 -1, -1, -1, -1, -1, -1, -1, -1,		/* 88-8f */
+		 -1, -1, -1, -1, -1, -1, -1, -1,		/* 90-97 */
+		 -1, -1, -1, -1, -1, -1, -1, -1,		/* 98-9f */
+		 -1, -1, -1, -1, -1, -1, -1, -1,		/* a0-a7 */
+		 -1, -1, -1, -1, -1, -1, -1, -1,		/* a8-af */
+		 -1, -1, -1, -1, -1, -1, -1, -1,		/* b0-b7 */
+		 -1, -1, -1, -1, -1, -1, -1, -1,		/* b8-bf */
+		 -1, -1, -1, -1, -1, -1, -1, -1,		/* c0-c7 */
+		 -1, -1, -1, -1, -1, -1, -1, -1,		/* c8-cf */
+		 -1, -1, -1, -1, -1, -1, -1, -1,		/* d0-d7 */
+		 -1, -1, -1, -1, -1, -1, -1, -1,		/* d8-df */
+		 -1, -1, -1, -1, -1, -1, -1, -1,		/* e0-e7 */
+		 -1, -1, -1, -1, -1, -1, -1, -1,		/* e8-ef */
+		 -1, -1, -1, -1, -1, -1, -1, -1,		/* f0-f7 */
+		 -1, -1, -1, -1, -1, -1, -1, -1,		/* f8-ff */
+	};
+	return val[c];
 }
 
 int get_sha1_hex(const char *hex, unsigned char *sha1)