Ensure that files in git shed are read-only.

Still todo: ensure that file permissions are preserved when downloading
from a content store. Will require switching to using rsync, presumably.
Switch downloading from http to rsync.

- Allows us to preserve file permissions.
- Is more robust in general.
Merge pull request #1 from benjyw/read_only

Ensure that files in git shed are read-only.
Merge pull request #2 from benjyw/read_with_rsync

Switch downloading from http to rsync.