reinstall-packages-from arg parsing

[brigand]

• Operating system and version:

OSX

• nvm debug output:

$SHELL: /bin/zsh
$NVM_DIR: $HOME/.nvm
nvm current: v8.10.0
which node: $NVM_DIR/versions/node/v8.10.0/bin/node
which iojs: iojs not found
which npm: $NVM_DIR/versions/node/v8.10.0/bin/npm
npm config get prefix: $NVM_DIR/versions/node/v8.10.0
npm root -g: $NVM_DIR/versions/node/v8.10.0/lib/node_modules

• nvm ls output:

       v0.10.48
        v0.12.2
         v4.2.2
        v6.11.0
        v7.10.0
         v8.9.0
->      v8.10.0
         v9.3.0
         v9.5.0
         system
default -> 8 (-> v8.10.0)
io -> iojs (-> N/A)
node -> stable (-> v9.5.0) (default)
stable -> 9.5 (-> v9.5.0) (default)
iojs -> iojs- (-> N/A) (default)

• How did you install nvm? (e.g. install script in readme, Homebrew):

Script from readme.

• What steps did you perform?

Note: there's no = in the following command.

$ nvm install 8.4.0 --reinstall-packages-from 8.10.0
######################################################################## 100.0%
Checksums empty
Now using node v8.4.0 (npm v5.3.0

• What happened?

$ ls ~/.nvm/versions/node/v8.4.0/lib/node_modules
npm

• What did you expect to happen?

I expected it to either copy over the packages, or give an error because I didn't include the =.

cc @ljharb