Few will offer 40Gbps without charging a pretty penny, generally the jump will go from 10Gbps to 100Gbps but not until it becomes cost-effective (and not anytime soon).

That said, SoftLayer does provide 20Gbps access within a private rack and 20Gbps access to the public network.

Most large scale operations are (or soon will be) deploying 25GE and/or 50GE in place of 10Gbps Ethernet. 100GE to each node is unnecessary for most workloads & more importantly it's obscenely expensive and likely to remain so for at least 3 more years.

Architecturally I would honestly wonder why you need that. Git, with its heavy reliance on immutable objects, should replicate very well. I would expect to be NIC-bound, but I would also expect it to horizontally scale reasonably well. Storage is obviously a concern -- you will have to make sure you don't end up needing a full copy of your data on every node -- but there are well known solutions to this problem that account for hot keys/objects well enough for you to get really really far. Even reaching as far back as the BigTable paper there is valuable stuff to look at and you can obviously look at Cassandra to see how the OSS world has tackled that problem.