gitserver: clone a repository from one instance to another

Created by: asdine

https://github.com/sourcegraph/sourcegraph/pull/26567 added the MigrateFrom option to clone a repository from another shard instead of cloning it from the code host. However, AFAIK the MigrateFrom option is not used by gitserver.

Add support for that option to gitserver to effectively clone the repo from the provided gitserver instance.