use @NotNull annotation for getHistory()

Author: Vladimir Kotal

opengrok-indexer/pom.xml

@@ -215,6 +215,11 @@ Portions Copyright (c) 2020-2020, Lubos Kosco <[email protected]>.
             <artifactId>micrometer-registry-statsd</artifactId>
             <version>${micrometer.version}</version>
         </dependency>
+        <dependency>
+            <groupId>org.jetbrains</groupId>
+            <artifactId>annotations</artifactId>
+            <version>20.1.0</version>
+        </dependency>
     </dependencies>
 
     <build>

opengrok-indexer/src/main/java/org/opengrok/indexer/history/Repository.java

@@ -51,6 +51,8 @@
 import org.opengrok.indexer.util.BufferSink;
 import org.opengrok.indexer.util.Executor;
 
+import org.jetbrains.annotations.NotNull;
+
 /**
  * An interface for an external repository.
  *
@@ -95,6 +97,7 @@ public abstract class Repository extends RepositoryInfo {
      *
      * @return {@code true} if the repository can get history for directories
      */
+    @NotNull
     abstract boolean hasHistoryForDirectories();
 
     /**