Let WorkspaceCommandHelper clone it. WorkspaceCommandHelper could return
workspace_id by reference, but doing that would introduce noisy .clone()
calls and lifetime mess.
In many of these places, we don't need an owned value, so using a
reference means we don't force the caller to clone the value. I really
doubt it will have any noticeable impact on performance (I think these
are all once-per-repo paths); it's just a little simpler this way.
`wc_commit` seems clearer than `checkout` and not too much longer. I
considered `working_copy` but it was less clear (could be the path to
the working copy, or an instance of `WorkingCopy`). I also considered
`working_copy_commit`, but that seems a bit too long.
We depend on comparing the workspace root with the Git repo's path to
know if we're sharing the working copy with it. For that to work
reliably, we need the paths to be canonicalized, so that's what this
patch tries to do.
Most tests need a repo but don't need a working copy. Let's have a
function for setting up a test repo. But first, let's free up the name
`init_repo()` by renaming it to `init_workspace()` (which is also more
accurate).
