> They most likely do not use standard git on the server side,
We don't use git-core, no. :-)
> but store the repository in big table
Almost. We store some information in BigTable, and most data directly in Google's filesystem. The bulk of the data is actually in relatively normal Git pack files, but they are stored differently than git-core would do.
> and have their own git implementation.
Not really. We use JGit (http://www.eclipse.org/jgit) with the DFS storage package (org.eclipse.jgit.storage.dfs) and some glue to connect that code to BigTable and the Google filesystem. We have no custom patches to JGit, everything was upstreamed already months ago.
We haven't yet figured out how to open source the glue code. Its non-trivial in size, which indicates the stock org.eclipse.jgit.storage.dfs package is not sufficient on its own to run a service like this. But most of the glue code calls Google specific APIs, like the BigTable client library. We could port this code to another database, but that would just be code thrown over the wall. What is in JGit right now is at least exactly what we run, and thus something we maintain and use every day.