sha1_name: convert uses of 40 to GIT_SHA1_HEXSZ

There are several uses of the constant 40 in find_unique_abbrev_r.
Convert them to GIT_SHA1_HEXSZ.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1_name.c

@@ -500,10 +500,10 @@ int find_unique_abbrev_r(char *hex, const unsigned char *sha1, int len)
 	}
 
 	sha1_to_hex_r(hex, sha1);
-	if (len == 40 || !len)
-		return 40;
+	if (len == GIT_SHA1_HEXSZ || !len)
+		return GIT_SHA1_HEXSZ;
 	exists = has_sha1_file(sha1);
-	while (len < 40) {
+	while (len < GIT_SHA1_HEXSZ) {
 		struct object_id oid_ret;
 		status = get_short_oid(hex, len, &oid_ret, GET_OID_QUIETLY);
 		if (exists