[error prone] fix "FutureReturnValueIgnored" error in FS

Change-Id: I53731091b3e34ac2e93a18f0ad6dd04dc56f8177
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/util/FS.java

@@ -363,7 +363,7 @@ public abstract class FS {
 							}
 							return attributes;
 						});
-				f.exceptionally(e -> {
+				f = f.exceptionally(e -> {
 					LOG.error(e.getLocalizedMessage(), e);
 					return Optional.empty();
 				});