feat(fs/git): add support for cloning target repository to the local filesystem

[GeorgeMac]

This adds support for having the Git backend clone to local disk.
The motivation is to offload some of the memory pressure for large repositories onto disk.
This way memory doesn't have to scale in proportion to repository size, with the tradeoff that initial clone and fetch times may be increased due to I/O on local disk.

storage:
  type: git
  git:
    backend:
      type: filesytem
      path: /path/to/dir # optional: will create a tempdir in systems temp dir location otherwise

[markphelps]

nice!