Enable access to context by DistCp subclasses

In the DistCp class, the context is private with no getter method allowing retrieval by subclasses. So a subclass would need to save its own copy of the inputOptions supplied to its constructor and reconstruct the context if it wishes to override the createInputFileListing method with logic similar to the original implementation, i.e. calling CopyListing#buildListing with a path and context.

I propose adding to DistCp this method,

  protected DistCpContext getContext() {
    return context;
  }