fix signed range problems with hex conversions

fix signed range problems with hex conversions

Make hexval_table[] "const".  Also make sure that the accessor
function hexval() does not access the table with out-of-range
values by declaring its parameter "unsigned char", instead of
"unsigned int".

With this, gcc can just generate:

	movzbl  (%rdi), %eax
	movsbl  hexval_table(%rax),%edx
	movzbl  1(%rdi), %eax
	movsbl  hexval_table(%rax),%eax
	sall    $4, %edx
	orl     %eax, %edx

for the code to generate a byte from two hex characters.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>