Add Git#shutdown for releasing resources held by JGit process

The shutdown method releases
- ThreadLocal held by NLS
- GlobalBundleCache used by NLS
- Executor held by WorkQueue

Bug: 437855
Bug: 550529
Change-Id: Icfdccd63668ca90c730ee47a52a17dbd58695ada
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/api/Git.java

 import org.eclipse.jgit.lib.Repository;
 import org.eclipse.jgit.lib.RepositoryBuilder;
 import org.eclipse.jgit.lib.RepositoryCache;
+import org.eclipse.jgit.lib.internal.WorkQueue;
+import org.eclipse.jgit.nls.NLS;
 import org.eclipse.jgit.util.FS;
 
 /**
 		return new InitCommand();
 	}
 
+	/**
+	 * Shutdown JGit and release resources it holds like NLS and thread pools
+	 * @since 5.8
+	 */
+	public static void shutdown() {
+		WorkQueue.getExecutor().shutdownNow();
+		NLS.clear();
+	}
+
 	/**
 	 * Construct a new {@link org.eclipse.jgit.api.Git} object which can
 	 * interact with the specified git repository.

org.eclipse.jgit/src/org/eclipse/jgit/nls/GlobalBundleCache.java

 			throw new Error(e);
 		}
 	}
+
+	static void clear() {
+		cachedBundles.clear();
+	}
 }

org.eclipse.jgit/src/org/eclipse/jgit/nls/NLS.java

 		return b.get(type);
 	}
 
+	/**
+	 * Release resources held by NLS
+	 * @since 5.8
+	 */
+	public static void clear() {
+		local.remove();
+		GlobalBundleCache.clear();
+	}
+
 	private final Locale locale;
 	private final ConcurrentHashMap<Class, TranslationBundle> map = new ConcurrentHashMap<>();