index-pack, unpack-objects: use skip_prefix to avoid magic number

index-pack, unpack-objects: use skip_prefix to avoid magic number

When parsing --pack_header=, we manually skip 14 bytes to the data.
Let's use skip_prefix() to do this automatically.

Note that we overwrite our pointer to the front of the string, so we
have to add more context to the error message. We could avoid this by
declaring an extra pointer to hold the value, but I think the modified
message is actually preferable; it should give translators a bit more
context.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>