Don't call reader.close() 2 times on dispose()

Bug: 479406
Change-Id: I6645a8f36ea349a5f04fd14d2c1ef2ecac2bcc37
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>

org.eclipse.jgit/src/org/eclipse/jgit/revwalk/RevWalk.java

@@ -1295,7 +1295,6 @@ public class RevWalk implements Iterable<RevCommit>, AutoCloseable {
 		retainOnReset = 0;
 		carryFlags = UNINTERESTING;
 		objects.clear();
-		reader.close();
 		roots.clear();
 		queue = new DateRevQueue();
 		pending = new StartGenerator(this);