The change to an array of readers comes from e096060b
(refactor(core/commands2/add) split loop, 2014-11-06), where it's used
to setup readers for each path in the argument list.  However, since
6faeee83 (cmds2/add: temp fix for -r. horrible hack, 2014-11-11) the
argument looping moved outside of add() and into Run(), so we can drop
the multiple-reader support from add().

Adding a file can create multiple nodes (e.g. the splitter can chunk
the file into several blocks), but:

1. we were only appending a single node per reader to our returned
   list, and
2. we are only using the final node in that returned list,

so this commit also adjusts add() to return a single node reference
instead on an array of nodes.