Use gitfs backed by the saltfs repo (https://github.com/servo/saltfs/)
as the main source for the Salt file tree.

For local testing, additionally configure a local directory in /tmp as
a Salt file tree root, and give it precedence over the gitfs tree.
This allows temporarily cloning the saltfs repo into that directory
and makes local changes to iterate on them.
Add an ADMIN_README file with some ground rules about proper usage of
the directory; this file should be set as immutable, but Salt does not
yet have a module/state for this.