[JENKINS-15493] Linking retroactively in changelog.

changelog.html

@@ -120,6 +120,7 @@ Upcoming changes</a>
     (<a href="https://issues.jenkins-ci.org/browse/JENKINS-15293">issue 15293</a>)
   <li class='rfe'>
     Memory footprint improvement, especially under large HTTP request threads.
+    (<a href="https://issues.jenkins-ci.org/browse/JENKINS-15493">issue 15493</a>)
   <li class='major rfe'>
     Build records are now lazy loaded, resulting in a reduced startup time 
     (<a href="https://issues.jenkins-ci.org/browse/JENKINS-8754">issue 8754</a>)