LogCommand.all(): filter out refs that do not refer to commit objects

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Change-Id: I48b1828e0b1304f76276ec07ebac7ee9f521b194

org.eclipse.jgit.test/tst/org/eclipse/jgit/api/LogCommandTest.java

@@ -100,11 +100,17 @@ public class LogCommandTest extends RepositoryTestCase {
 		commits.add(git.commit().setMessage("initial commit").call());
 
 		TagCommand tagCmd = git.tag();
-		tagCmd.setName("tagname");
+		tagCmd.setName("tagcommit");
 		tagCmd.setObjectId(commits.get(0));
 		tagCmd.setTagger(new PersonIdent(db));
 		Ref tag = tagCmd.call();
 
+		tagCmd = git.tag();
+		tagCmd.setName("tagtree");
+		tagCmd.setObjectId(commits.get(0).getTree());
+		tagCmd.setTagger(new PersonIdent(db));
+		tagCmd.call();
+
 		Iterator<RevCommit> log = git.log().all().call().iterator();
 		assertTrue(log.hasNext());
 		RevCommit commit = log.next();

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

@@ -241,10 +241,23 @@ public class LogCommand extends GitCommand<Iterable<RevCommit>> {
 		for (Ref ref : getRepository().getAllRefs().values()) {
 			if(!ref.isPeeled())
 				ref = getRepository().peel(ref);
+
 			ObjectId objectId = ref.getPeeledObjectId();
 			if (objectId == null)
 				objectId = ref.getObjectId();
-			add(objectId);
+			RevCommit commit = null;
+			try {
+				commit = walk.parseCommit(objectId);
+			} catch (MissingObjectException e) {
+				// ignore: the ref points to an object that does not exist;
+				// it should be ignored as traversal starting point.
+			} catch (IncorrectObjectTypeException e) {
+				// ignore: the ref points to an object that is not a commit
+				// (e.g. a tree or a blob);
+				// it should be ignored as traversal starting point.
+			}
+			if (commit != null)
+				add(commit);
 		}
 		return this;
 	}