use skip_prefix() to avoid more magic numbers

Continue where ae021d87 (use skip_prefix to avoid magic numbers) left off
and use skip_prefix() in more places for determining the lengths of prefix
strings to avoid using dependent constants and other indirect methods.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/cat-file.c

@@ -82,8 +82,9 @@ static int cat_one_file(int opt, const char *exp_type, const char *obj_name)
 				enum object_type type;
 				unsigned long size;
 				char *buffer = read_sha1_file(sha1, &type, &size);
-				if (memcmp(buffer, "object ", 7) ||
-				    get_sha1_hex(buffer + 7, blob_sha1))
+				const char *target;
+				if (!skip_prefix(buffer, "object ", &target) ||
+				    get_sha1_hex(target, blob_sha1))
 					die("%s not a valid tag", sha1_to_hex(sha1));
 				free(buffer);
 			} else