Similar to previous issues around doing way too much verification at loading
time, checking whether the object index chunk is monotonic is better left for
git-fsck instead of every single time we want to look at something in the
repository.

As midx files grow, this starts taking more and more times. As an example, I
went looking for this because it's taking about 1.5s to do a single object
lookup in a repository that's ended up with a 7G multi-pack-index file.