[Performance] make auto-using faster.
[Performance] make auto-using faster.

Per https://github.com/creationix/nvm/issues/539#issuecomment-236830764
creationix/nvm,creationix/nvm,ljharb/nvm,evocateur/nvm,edwmurph/nvm,lukechilds/nvm,PeterDaveHelloKitchen/nvm,PeterDaveHello/nvm,ljharb/nvm,edwmurph/nvm,lukechilds/nvm,PeterDaveHelloKitchen/nvm,evocateur/nvm,PeterDaveHello/nvm