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.
Merge pull request #3042 from gourlaysama/t7634-repl-sh-is-broken

SI-7634 resurrect the REPL's :sh command