Add a context parameter to listDirFn so that each call site can pass
its own context. The closures in makeListDir previously captured
m.Ctx at creation time; they now use the context passed at call time
instead. This is needed so that processJob can pass a cancellable
context for the destination listing independently of the source.
Note: callers must pass m.Ctx (or a child of it) to preserve the
existing cancellation behaviour where listings stop when the march
context is cancelled.
Nick Craig-Wood
2228e7c866