rbenv(): preserve multiline output of `sh-*` commands
Fix multiple argument completion with zsh

This changes the zsh completion to omit the final, incomplete command
line argument when invoking rbenv completions, making it consistent with
the bash completion.  Since no built-in completion cares about the
argument list, this inconsistency only affected plugins.
Merge remote-tracking branch 'sstephenson/master'

* sstephenson/master:
  Fix multiple argument completion with zsh
  rbenv(): preserve multiline output of `sh-*` commands