I think it's better to let the caller decide if the parents should be
added. One use case for removing a head is when fetching from a Git
remote where a branch has been rewritten. In that case, it's probably
the best user experience to remove the old head. With the current
semantics of `View::remove_head()`, we would need to walk up the graph
to find a commit that's an ancestor and for each commit we remove as
head, its parents get temporarily added as heads. It's much easier for
callers that want to add the parents as heads to do that.
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`).
