HSEARCH-3517 Use fetchHits on fetchSingleHit

So that fetchSingleHit will be optimized as well

engine/src/main/java/org/hibernate/search/engine/logging/impl/Log.java

@@ -236,8 +236,8 @@ SearchException unableToParseTemporal(@FormatWith(SimpleNameClassFormatter.class
 	@Message(id = ID_OFFSET_2 + 60, value = "Invalid '%1$s' value for enum '%2$s'.")
 	SearchException invalidStringForEnum(String value, @FormatWith(ClassFormatter.class) Class<? extends Enum> enumType, @Cause Exception cause);
 
-	@Message(id = ID_OFFSET_2 + 61, value = "Multiple hits when a single hit was expected: got %1$s hits.")
-	SearchException nonSingleHit(long totalHitCount);
+	@Message(id = ID_OFFSET_2 + 61, value = "Multiple hits when a single hit was expected.")
+	SearchException nonSingleHit();
 
 	@Message(id = ID_OFFSET_2 + 62,
 			value = "The thread was interrupted while a work was being submitted to '%1$s'."

engine/src/main/java/org/hibernate/search/engine/search/query/spi/AbstractSearchQuery.java

@@ -61,14 +61,13 @@ public List<H> fetchHits(Integer offset, Integer limit) {
 	@Override
 	public Optional<H> fetchSingleHit() {
 		// We don't need to fetch more than two elements to detect a problem
-		R result = fetch( 2 );
-		List<H> hits = result.hits();
-		int fetchedHitCount = result.hits().size();
+		List<H> hits = fetchHits( 2 );
+		int fetchedHitCount = hits.size();
 		if ( fetchedHitCount == 0 ) {
 			return Optional.empty();
 		}
 		else if ( fetchedHitCount > 1 ) {
-			throw log.nonSingleHit( result.totalHitCount() );
+			throw log.nonSingleHit();
 		}
 		else {
 			return Optional.of( hits.get( 0 ) );