use an in-memory cache so branches don't get downloaded twice

[matthewmueller]

Since we're not caching branches like master, we're fetching these packages twice.

[frankwallis]

I'm not sure if this is related but I believe this is an error:

Package.prototype.fetch = unyield(function *() {

  // resolve
  var ref = this.resolved || (yield this.resolve());
  var token = this.tok ? this.tok + '@' : '';
  var url = fmt('https://%sgithub.com/%s/archive/%s.tar.gz', token, this.repo, ref);
  var cache = join(cachepath, this.slug() + '.tar.gz');
  var dest = this.path();

  // inflight, wait till other package completes
  if (inflight[dest]) {
    var pkg = inflight[dest];
    yield function(done){ pkg.once('fetch', done); };
  }

  // set package as inflight
  inflight[dest] = this;

  // check if directory already exists
  if (yield this.exists()) {

If two things are waiting on the event emitter then when it fires they will both run at the same time. I think it needs to use while:

// inflight, wait till other package completes
  while (inflight[dest]) {
    var pkg = inflight[dest];
    yield function(done){ pkg.once('fetch', done); };
  }