Run update steps only if they have not been run before

scm-core/src/main/java/sonia/scm/migration/UpdateStep.java

@@ -1,10 +1,13 @@
 package sonia.scm.migration;
 
 import sonia.scm.plugin.ExtensionPoint;
+import sonia.scm.version.Version;
 
 @ExtensionPoint
 public interface UpdateStep {
   void doUpdate();
 
-  String getTargetVersion();
+  Version getTargetVersion();
+
+  String affectedDataType();
 }