This commits makes it so that running commands outside a repo results
in an error message instead of a panic.
We still don't look for a `.jj/` directory in ancestors of the current
directory.
I'm preparing to publish an early version before someone takes the
name(s) on crates.io. "jj" has been taken by a seemingly useless
project, but "jujube" and "jujube-lib" are still available, so let's
use those.
This removes one level of indirection, which is nice because it was
visible to the callers. The `Index` struct is now empty. The next step
is obviously to delete it (and perhaps rename `IndexFile` to `Index`
or `ReadonlyIndex`).
