Fix put to cache check

putIfAbsent should return null, whenever there was no value for the key
before it was put with the call. But however this does not seem to work
with the concurrent hash map (liskov?)

scm-core/src/main/java/sonia/scm/repository/util/CachingAllWorkdirProvider.java

@@ -79,7 +79,8 @@ public class CachingAllWorkdirProvider implements CacheSupportingWorkdirProvider
   @Override
   public void contextClosed(CreateWorkdirContext<?, ?, ?> createWorkdirContext, File workdir) throws IOException {
     String id = createWorkdirContext.getScmRepository().getId();
-    if (workdirs.putIfAbsent(id, workdir) != null) {
+    File putResult = workdirs.putIfAbsent(id, workdir);
+    if (putResult != null && putResult != workdir) {
       deleteWorkdir(workdir);
     }
   }