function getShaFromPullRequest(user, repo, number, callback) {
  github.pullRequests.get({user, repo, number}, (error, res) => {
    if (error) {
      console.log(error);
      return;
    }
    callback(res.head.sha);
  });
}
There is unfortunately no standard format to report an error, so we have
to write a specific converter for each tool we want to support.

Those functions take a json object as input and fill the output with the
following format:

{ [ path: string ]: Array< { message: string, line: number }> }

This is an object where the keys are the path of the files and values
is an array of objects of the shape message and line.