Move all cd commands into sub commands

This is to stop the user's CWD being changed

zsh-nvm.plugin.zsh

@@ -22,22 +22,19 @@ _zsh_nvm_latest_release_tag() {
 
 _zsh_nvm_install() {
   git clone https://github.com/creationix/nvm.git "$NVM_DIR"
-  cd "$NVM_DIR"
-  git checkout --quiet "$(_zsh_nvm_latest_release_tag)"
+  $(cd "$NVM_DIR" && git checkout --quiet "$(_zsh_nvm_latest_release_tag)")
 }
 
 nvm_update() {
-  cd "$NVM_DIR"
   echo "Checking latest version of nvm..."
-  local installed_version=$(git describe --tags)
+  local installed_version=$(cd "$NVM_DIR" && git describe --tags)
   local latest_version=$(_zsh_nvm_latest_release_tag)
   echo "Installed version is $installed_version"
   if [[ "$installed_version" = "$latest_version" ]]; then
     echo "You're already up to date"
   else
     echo "Updating to $latest_version..."
-    git fetch
-    git checkout "$latest_version"
+    $(cd "$NVM_DIR" && git fetch && git checkout "$latest_version")
   fi
 }