[spotbugs] DfsReftableDatabase: extract lock to local variable

This fixes UL_UNRELEASED_LOCK_EXCEPTION_PATH raised by spotbugs in
#DfsReftableDatabase and #clearCache.

Change-Id: Ifd3189288d2a8e64139c02cd105eb335fa2f68cf
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsReftableDatabase.java

 import java.util.Map;
 import java.util.Set;
 import java.util.TreeSet;
+import java.util.concurrent.locks.Lock;
 import java.util.concurrent.locks.ReentrantLock;
 
 import org.eclipse.jgit.annotations.Nullable;
 		reftableDatabase = new ReftableDatabase() {
 			@Override
 			public MergedReftable openMergedReftable() throws IOException {
-				DfsReftableDatabase.this.getLock().lock();
+				Lock l = DfsReftableDatabase.this.getLock();
+				l.lock();
 				try {
 					return new MergedReftable(stack().readers());
 				} finally {
-					DfsReftableDatabase.this.getLock().unlock();
+					l.unlock();
 				}
 			}
 		};
 
 	@Override
 	void clearCache() {
-		getLock().lock();
+		ReentrantLock l = getLock();
+		l.lock();
 		try {
 			if (ctx != null) {
 				ctx.close();
 				stack = null;
 			}
 		} finally {
-			getLock().unlock();
+			l.unlock();
 		}
 	}