packfile.c: protect against disappearing indexes

In 17c35c8969 (packfile: skip loading index if in multi-pack-index,
2018-07-12) we stopped loading the .idx file for packs that are
contained within a multi-pack index.

This saves us the effort of loading an .idx and doing some lightweight
validity checks by way of 'packfile.c:load_idx()', but introduces a race
between processes that need to load the index (e.g., to generate a
reverse index) and processes that can delete the index.

For example, running the following in your shell:

    $ git init repo && cd repo
    $ git commit --allow-empty -m 'base'
    $ git repack -ad && git multi-pack-index write

followed by:

    $ rm -f .git/objects/pack/pack-*.idx
    $ git rev-parse HEAD | git cat-file --batch-check='%(objectsize:disk)'

will result in a segfault prior to this patch. What's happening here is
that we notice that the pack is in the multi-pack index, and so don't
check that it still has a .idx. When we then try and load that index to
generate a reverse index, we don't have it, so the call to
'find_pack_revindex()' in 'packfile.c:packed_object_info()' returns
NULL, and then dereferencing it causes a segfault.

Of course, we don't ever expect someone to remove the index file by
hand, or to be in a state where we never wrote it to begin with (yet
find that pack in the multi-pack-index). But, this can happen in a
timing race with 'git repack -ad', which removes all existing packs
after writing a new pack containing all of their objects.

Avoid this by reverting the hunk of 17c35c8969 which stops loading the
index when the pack is contained in a MIDX. This makes the latter half
of 17c35c8969 useless, since we'll always have a non-NULL
'p->index_data', in which case that if statement isn't guarding
anything.

These two together effectively revert 17c35c8969, and avoid the race
explained above.

Co-authored-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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