`ReadonlyRepo::init_*()` currently calls `WorkingCopy::init()`. In
order to remove that dependency, this patch wraps the
`ReadonlyRepo::init_*()` functions in new `Workspace` functions. A
later patch will have those functions call `WorkspaceCopy::init()`.`
The `Repo` doesn't do anything with the `WorkingCopy` except keeping a
reference to it for its users to use. In fact, the entire lib crate
doesn't do antyhing with the `WorkingCopy`. It therefore seems simpler
to have the users of the crate manage the `WorkingCopy` instance. This
patch does that by letting `Workspace` own it. By not keeping an
instance in `Repo`, which is `Sync`, we can also drop the
`Arc<Mutex<>>` wrapping.
I left `Repo::working_copy()` for convenience for now, but now it
creates a new instance every time. It's only used in tests.
This further decoupling should help us add support for multiple
working copies (#13).
Having a concept of a "workspace" will be useful for adding support
for multiple workspaces (#13). You can think of the "workspace" as a
repo combined with a working copy. A workspace corresponds 1:1 with a
`.jj/` directory. It's pretty close to what other VCS simply call a
"repo", but I've ended up using the word "repo" for what Git calls a
"bare repo".
