SI-7634 resurrect the REPL's :sh command

ProcessResult had a companion object in 2.10 that somehow disappeared in
2.11. It only called "new ProcessResult(...)", so the REPL might just as
well do that.

---
This regression happened because `:sh` isn't tested; but then, I am not sure how to write a test for this that isn't platform-dependent.

Review by @som-snytt