small optimization for global facet with facet filter, by removing the double check on the facet filter

src/main/java/org/elasticsearch/search/facet/AbstractFacetCollector.java

@@ -48,6 +48,12 @@ public abstract class AbstractFacetCollector extends FacetCollector {
         return this.filter;
     }
 
+    public Filter getAndClearFilter() {
+        Filter filter = this.filter;
+        this.filter = null;
+        return filter;
+    }
+
     @Override
     public void setFilter(Filter filter) {
         if (this.filter == null) {

src/main/java/org/elasticsearch/search/facet/FacetPhase.java

@@ -103,7 +103,9 @@ public class FacetPhase implements SearchPhase {
                     if (collector instanceof AbstractFacetCollector) {
                         AbstractFacetCollector facetCollector = (AbstractFacetCollector) collector;
                         if (facetCollector.getFilter() != null) {
-                            filter = facetCollector.getFilter();
+                            // we can clear the filter, since we are anyhow going to iterate over it
+                            // so no need to double check it...
+                            filter = facetCollector.getAndClearFilter();
                         }
                     }
                     List<Collector> list = filtersByCollector.get(filter);