When `rbenv init` is invoked to automatically edit shell startup files,
the generated lines will be invoking `rbenv init - --no-rehash` by default
to help speed up shell startup. Auto-rehashing on every shell startup can
be slow for some users but is not crucial to rbenv operation. Lets assume
that rbenv shims are already healthy and that they will be regenerated as
needed after installing new ruby versions and gems.
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