Redirect a GitHub URL to the last-committed version of that thing.

petrify.js

/*!
 * Petrify
 * Redirect yourself to a permanent version of a GitHub blob or tree.
 *
 * Say you're looking at `https://github.com/documentcloud/documentcloud/blob/master/public/javascripts/embed/search_embed.js#L315-L317`
 * and you want to link to it in an email or GitHub comment. You realize that,
 * when people follow the link years or even days later, it's likely the file
 * will have changed so much that those line references won't be the same, and
 * people won't know what you were talking about.
 * 
 * So you click your Petrify bookmarklet. It recognizes the last time that blob
 * or tree were changed and redirects you to the permanent version of that URL:
 * `https://github.com/documentcloud/documentcloud/blob/bb009c6875baedc7d58ed699e0b0ef02fc078ccd/public/javascripts/embed/search_embed.js#L315-L317`.
 * You can safely share or link to this URL, knowing that years from now, your
 * snoopy descendents can still know what you were talking about.
 *
 * To use, create a bookmark with the following address (sans backticks):
 * `javascript:!function(){var t=window.location.href,o=t.match("^https://github.com/([^/]{1,})/([^/]{1,})/(blob|tree)/([^/]{1,})?/?(.*)$");if(o){var e=document.querySelector("a.commit-tease-sha");if(e&&e.href){var a=e.href,i=a.substr(a.lastIndexOf("/")+1),r=["https://github.com",o[1],o[2],o[3],i];o[5]&&r.push(o[5]);var h=r.join("/");h!=t&&(window.location=h)}}}();`
 *
 * TODO:
 * - Copy URL into clipboard instead of redirecting (maybe? and let them know?)
 * - Make a GH page with link people can drag to their bookmark bar
 *
 * @license (c) 2015 Justin Reese, DocumentCloud
 * Petrify may be freely distributed under the MIT license.
 *
 */

var current = window.location.href;
var matches = current.match('^https:\/\/github.com\/([^/]{1,})\/([^/]{1,})\/(blob|tree)\/([^/]{1,})?\/?(.*)$');
if (matches) {
 /*
  * matches[0]: Original URL
  * matches[1]: Owner slug
  * matches[2]: Repo slug
  * matches[3]: `blob` or `tree`
  * matches[4]: Branch or commit hash
  * matches[5]: Anything following (optional)
  */
  var lastCommitLink = document.querySelector('a.commit-tease-sha');
  if (lastCommitLink && lastCommitLink.href) {
    var lastCommitURL = lastCommitLink.href;
    var lastHash      = lastCommitURL.substr(lastCommitURL.lastIndexOf('/') + 1);
    var components    = ['https://github.com', matches[1], matches[2], matches[3], lastHash];
    if (matches[5]) { components.push(matches[5]); }
    var petrified = components.join('/');
    if (petrified != current) {
      window.location = petrified;
    }
  }
}

Hey dummy, GitHub does this natively with permanent links. Just hit y on any file page! Dummy!