In the event that `eval "$(rbenv init -)"` is called from a function named
rbenv (which I do to get rbenv to load lazily in my shell), evaluating the
phrase `rbenv rehash` will cause the outer function to run again (causing an
infinite loop).
This change makes it clear you want the command named rbenv and not a function
which may exist in the environment.
c9a96c9f79