update changelog

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
-v 7.13.0
-v 7.14.0 (unreleased)
-  - Fix full screen mode for snippet comments (Daniel Gerhardt)
-  - Fix 404 error in files view after deleting the last file in a repository (Stan Hu)
-  - Fix label read access for unauthenticated users (Daniel Gerhardt)
-  - Fix access to disabled features for unauthenticated users (Daniel Gerhardt)
-  - Fix OAuth provider bug where GitLab would not go return to the redirect_uri after sign-in (Stan Hu)
-  - Fix file upload dialog for comment editing (Daniel Gerhardt)
-  - Set OmniAuth full_host parameter to ensure redirect URIs are correct (Stan Hu)
-  - Expire Rails cache entries after two weeks to prevent endless Redis growth
-  - Add support for destroying project milestones (Stan Hu)
-  - Add fetch command to the MR page.
-
 v 7.13.1
   - Revert issue caching
+  - Improve documentation
 
 v 7.13.0 (unreleased)
   - Remove repository graph log to fix slow cache updates after push event (Stan Hu)