[Fix] ensure all `grep` calls go through `nvm_grep`.
[Fix] ensure all `grep` calls go through `nvm_grep`.

Fixes #1615.
edwmurph/nvm,PeterDaveHelloKitchen/nvm,PeterDaveHello/nvm,creationix/nvm,evocateur/nvm,creationix/nvm,ljharb/nvm,edwmurph/nvm,evocateur/nvm,ljharb/nvm,PeterDaveHelloKitchen/nvm,PeterDaveHello/nvm