A snippet using GitHub's public API to get the URL and SHA of the first commit for a given repo. Works also for private repos, but you need a token.

first-commit.js

// A snippet using GitHub's public API to get
// the URL and SHA of the first commit for a given repo
function openFirstCommit(userorg, repo, authToken) {

  var opts = authToken ?
    { headers: new Headers({
        'Authorization': 'token ' + authToken
      })
    } : {}

  return fetch('https://api.github.com/repos/'+userorg+'/'+repo+'/commits', opts)
    .then(obj=>obj.headers.get('link')) 
      // the link header has additional urls for paging
  
    .then(link=>link.split(',')[1].split(';')[0].slice(2,-1))
      // the link contains two urls in the form 
      // <https://github.com/...>; rel=blah, <https://github.com/...>; rel=thelastpage
      // split the url out of the string
  
    .then(pageurl=>fetch(pageurl, opts).then(x=>x.json()))
      // open the url to the last page
  
    .then(commits=>commits[commits.length-1].html_url)
      // navigate to the last commit and extract the userpage url
  
    .then(x=>window.location = x);
      // navigate there
}


// Example usage
openFirstCommit('facebook', 'react');