server-info: drop objdirlen pointer arithmetic

server-info: drop objdirlen pointer arithmetic

When writing objects/info/packs, we use the basename of each pack
(i.e., just the "pack-1234abcd.pack" part). We compute that manually by
adding "objdirlen + 6" to the name.

This _should_ work consistently, as we do not include non-local packs,
meaning everything should be in $objdir/pack/. Before f13d7db4af
(server-info.c: use pack_local like everybody else., 2005-12-05), this
was definitely true, since we computed "local" based on comparing the
objdir string.  Since then, we're relying on the code on packfile.c to
match our expectations of p->pack_name and p->local.

I think our expectations do still hold today, but we can be a bit more
defensive by just using pack_basename() to get the base. That
future-proofs us, and should hopefully be more obviously safe to
somebody reading the code.

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