Fix 'Exception usage: PHABRICATOR_USER should be defined' during land to hosted git repo

The error appears for me on a git repo hosted over http,
while another user experiences the same problem with a
ssh-served repo:

https://github.com/phacility/phabricator/issues/566

The root cause is the following in phabricator/bin/commit-hook:

  $username = getenv(DiffusionCommitHookEngine::ENV_USER);
  if (!strlen($username)) {
    throw new Exception(
      pht('usage: %s should be defined!', DiffusionCommitHookEngine::ENV_USER));
  }