Get the contents of a file or directory in Git using this function on any web page.

getGitContents.js

/**
 * Gets the GitHub contents of a file or directory.
 * @param {string} user
 *   The GitHub user name.
 * @param {string} repo
 *   The name of the repo.  If you want to also specify the branch then you can
 *   append a colon to the repo name and then specify the branch name in this
 *   same field.
 * @param {string} path
 *   The path to the file or directory whose contents should be retrieved.
 * @param {function(*,*,Object)} callback
 *   The callback function that will be called with the contents of the GitHub
 *   file or directory.  The arguments passed to this callback will be the (1)
 *   contents found, (2) the data object, and (3) the meta object.  The first
 *   argument will be the base64 decoded content if the encoding used by GitHub
 *   was base64.  Otherwise the first argument will be the data specified by
 *   GitHub.  If the specified path is for a directory the first argument will
 *   be an array of objects representing the contents of the directory.
 */
function getGitContents(user, repo, path, callback) {
  let tmpName = ('_' + Math.random() + Math.random()).replace(/\./g, '');
  window[tmpName] = function (result) {
    delete window[tmpName];
    let { data, meta } = result;
    let contents = Array.isArray(data)
      ? data
      : data.encoding === 'base64'
        ? atob(data.content)
        : data;
    callback(contents, data, meta);
  };

  let branch = 'master';
  repo = repo.replace(/:([^:]+)/, function(match, branchName) {
    branch = branchName;
  });
  
  let url = `https://api.github.com/repos/${user}/${repo}/contents/`
    + path.replace(/[^\/]+/, encodeURIComponent)
    + `?callback=${tmpName}&ref=${encodeURIComponent(branch)}`;
  let script = document.createElement('script');
  script.src = url;
  (document.head || document.body).appendChild(script);
}

~example-branch.js

// Get the contents of the README.md file from westc's repo called grafana-vuehtml-panel on the master branch:
getGitContents('westc', 'grafana-vuehtml-panel:master', 'README.md', function(contents, data, meta) {
  console.log(contents);
});

~example-simple.js

// Get the contents of the README.md file from westc's repo called grafana-vuehtml-panel:
getGitContents('westc', 'grafana-vuehtml-panel', 'README.md', function(contents, data, meta) {
  console.log(contents);
});