When proxying an upload to a special remote, verify the hash.

When proxying an upload to a special remote, verify the hash.

While usually uploading to a special remote does not verify the content,
the content in a repository is assumed to be valid, and there is no trust
boundary. But with a proxied special remote, there may be users who are
allowed to store objects, but are not really trusted.

Another way to look at this is it's the equivilant of git-annex-shell
checking the hash of received data, which it does (see StoreContent
implementation).