[FIXED JENKINS-6747] support concurrent runs of matrix builds.

The actual fix was in c86aa679

changelog.html

@@ -61,6 +61,7 @@ Upcoming changes</a>
     Exposed plugin manager and update center to the REST API
   <li class=rfe>
     Enabled concurrent build support for matrix projects
+    (<a href="https://issues.jenkins-ci.org/browse/JENKINS-6747">issue 6747</a>)
 </ul>
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