function getFilesModifiedInBranch (branchName) {
  var promise = new Promise(function (resolve, reject) {
    var files;
    var command = 'git diff --name-only ' + branchName + ' `git merge-base ' + branchName + ' master`';

    child.exec(command, function (error, stdout, stderr) {
      if (!error) {
        var newLine = /(\r\n|\n|\r)/;
        files = stdout.split(newLine).filter(function (file) {
          return file.trim().length > 0;
        });
        files = files.map(function (file) {
          return path.resolve('.', file);
        });
        resolve(files);
      }
    });
  });

  return promise;
}
getFilesModifiedInBranch

@param {string} branchName The branch name to get the list of modified file list from.
@returns {Promise} result.
@resolves {string[]} The list of modified files in the given branch.