multi-pack-index: add 'verify' verb

multi-pack-index: add 'verify' verb

The multi-pack-index builtin writes multi-pack-index files, and
uses a 'write' verb to do so. Add a 'verify' verb that checks this
file matches the contents of the pack-indexes it replaces.

The current implementation is a no-op, but will be extended in
small increments in later commits.

Signed-off-by: Derrick Stolee <redacted>
Signed-off-by: Junio C Hamano <redacted>