add tests for indexing packs with delta cycles

If we receive a broken or malicious pack from a remote, we
will feed it to index-pack. As index-pack processes the
objects as a stream, reconstructing and hashing each object
to get its name, it is not very susceptible to doing the
wrong with bad data (it simply notices that the data is
bogus and aborts).

However, one question raised on the list is whether it could
be susceptible to problems during the delta-resolution
phase. In particular, can a cycle in the packfile deltas
cause us to go into an infinite loop or cause any other
problem?

The answer is no.

We cannot have a cycle of delta-base offsets, because they
go only in one direction (the OFS_DELTA object mentions its
base by an offset towards the beginning of the file, and we
explicitly reject negative offsets).

We can have a cycle of REF_DELTA objects, which refer to
base objects by sha1 name. However, index-pack does not know
these sha1 names ahead of time; it has to reconstruct the
objects to get their names, and it cannot do so if there is
a delta cycle (in other words, it does not even realize
there is a cycle, but only that there are items that cannot
be resolved).

Even though we can reason out that index-pack should handle
this fine, let's add a few tests to make sure it behaves
correctly.

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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