Add 8.6.4 changelog item

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.7.0 (unreleased)
-  - Don't attempt to fetch any tags from a forked repo (Stan Hu)
   - Improved Markdown rendering performance !3389 (Yorick Peterse)
   - Don't attempt to look up an avatar in repo if repo directory does not exist (Stan hu)
   - Preserve time notes/comments have been updated at when moving issue
@@ -20,6 +19,9 @@ v 8.7.0 (unreleased)
   - Fall back to `In-Reply-To` and `References` headers when sub-addressing is not available (David Padilla)
   - Remove "Congratulations!" tweet button on newly-created project. (Connor Shea)
 
+v 8.6.4
+  - Don't attempt to fetch any tags from a forked repo (Stan Hu)
+
 v 8.6.3
   - Mentions on confidential issues doesn't create todos for non-members. !3374
   - Destroy related todos when an Issue/MR is deleted. !3376