When verifying if a multi-pack-index file is valid, we want the
command to fail to signal an invalid file. Previously, we wrote
an error to stderr and continued as if we had no multi-pack-index.
Now, die() instead of error().
Add tests that check corrupted headers in a few ways:
* Bad signature
* Bad file version
* Bad hash version
* Truncated hash count
* Extended hash count
Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>