Don't allow a new layer to be created if it matches the existing state.

src/main/java/org/qortal/arbitrary/ArbitraryDataDiff.java

@@ -49,6 +49,7 @@ public class ArbitraryDataDiff {
             this.hashPreviousState();
             this.findAddedOrModifiedFiles();
             this.findRemovedFiles();
+            this.validate();
             this.writeMetadata();
 
         } finally {
@@ -222,6 +223,12 @@ public class ArbitraryDataDiff {
         }
     }
 
+    private void validate() {
+        if (this.addedPaths.isEmpty() && this.modifiedPaths.isEmpty() && this.removedPaths.isEmpty()) {
+            throw new IllegalStateException("Current state matches previous state. Nothing to do.");
+        }
+    }
+
     private void writeMetadata() throws IOException {
         ArbitraryDataMetadataPatch metadata = new ArbitraryDataMetadataPatch(this.diffPath);
         metadata.setAddedPaths(this.addedPaths);