t5324: harmonize sha1/sha256 graph chain corruption

In t5324.20, we corrupt a hex character 60 bytes into the graph chain file. Since the file consists of two hash identifiers, one per line, the corruption differs between sha1 and sha256. In a sha1 repository, the corruption is on the second line, and in a sha256 repository, it is on the first. We should of course detect the problem with either line. But as the next few patches will show (and fix), that is not the case (in fact, we currently do not exit non-zero for either line!). And while at the end of our series we'll catch all errors, our intermediate states will have differing behavior between the two hashes. Let's make sure we test corruption of both the first and second lines, and do so consistently with either hash by choosing offsets which are always in the first hash (30 bytes) or in the second (70). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>