Prevent NPE if ref can't be resolved when executing ReflogCommand

If the specified ref can't be resolved we should throw a
RefNotFoundException instead of an NPE.

Bug: 455005
Change-Id: I6ec9bf1c0f330eea5eb8277268f62663bdf58f66
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

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

@@ -48,6 +48,7 @@ import java.util.Collection;
 
 import org.eclipse.jgit.api.errors.GitAPIException;
 import org.eclipse.jgit.api.errors.InvalidRefNameException;
+import org.eclipse.jgit.api.errors.RefNotFoundException;
 import org.eclipse.jgit.internal.JGitText;
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.ReflogEntry;
@@ -97,6 +98,9 @@ public class ReflogCommand extends GitCommand<Collection<ReflogEntry>> {
 
 		try {
 			ReflogReader reader = repo.getReflogReader(ref);
+			if (reader == null)
+				throw new RefNotFoundException(MessageFormat.format(
+						JGitText.get().refNotResolved, ref));
 			return reader.getReverseEntries();
 		} catch (IOException e) {
 			throw new InvalidRefNameException(MessageFormat.format(