lib/sysrev/sysrev.js
function(systemId, revisionId, target, cb) {
    var repoPath = _meta.repoPath(systemId);
    git.getFileRevision(repoPath, revisionId, target + '.json', function(err, rev) {
      if (err) { return cb(err); }

      var s;
      try {
        s = JSON.parse(rev);
      }
      catch (e) {
        return cb(new Error('invalid system definition: ' + e.message), null);
      }
      cb(err, s);
    });
  }
function(systemId, revisionId, target, cb) {
    var repoPath = _meta.repoPath(systemId);
    git.getFileRevision(repoPath, revisionId, target + '.json', function(err, rev) {
      if (err) { return cb(err); }

      var s;
      try {
        s = JSON.parse(rev);
      }
      catch (e) {
        return cb(new Error('invalid system definition: ' + e.message), null);
      }
      cb(err, s);
    });
  }
get a revision from the history, with a specific version number