Update hermit-fetch. Add more testing.

Make `hermit-fetch` respond appropriately when missing required arguments. Write tests for init and fetch that do not depend on installation script.
Should the `FETCH_REPO` really be another copy of hermit? Fetch should be pulling down a git repo that just contains someone's home directory stuff like [my home repo](https://github.com/RadicalZephyr/home).

Though it is somewhat strange to pull in random other repo's, I think this is probably the right thing to do.