multi-pack-index: add builtin

This new 'git multi-pack-index' builtin will be the plumbing access
for writing, reading, and checking multi-pack-index files. The
initial implementation is a no-op.

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