[PATCH] Warning fix for gcc 4

This patch fixes the only warning reported by gcc 4.0.1 on Fedora Core 4
for x86_64:

sha1_file.c:1391: warning: pointer targets in assignment differ in
signedness

Signed-off-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

sha1_file.c

@@ -1388,7 +1388,7 @@ int write_sha1_from_fd(const unsigned char *sha1, int fd, char *buffer,
 		ssize_t size;
 		if (*bufposn) {
 			stream.avail_in = *bufposn;
-			stream.next_in = buffer;
+			stream.next_in = (unsigned char *) buffer;
 			do {
 				stream.next_out = discard;
 				stream.avail_out = sizeof(discard);