multi-pack-index: verify packname order

multi-pack-index: verify packname order

The final check we make while loading a multi-pack-index is that
the packfile names are in lexicographical order. Make this error
be a die() instead.

In order to test this condition, we need multiple packfiles.
Earlier in t5319-multi-pack-index.sh, we tested the interaction with
'git repack' but this limits us to one packfile in our object dir.
Move these repack tests until after the 'verify' tests.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>