Fix PlotCommit for commits with duplicate parents

JGit allows to create commits which have duplicate parents: e.g. a
commit X has first parent Y and second parent Y. Such commits are not
handled correctly by PlotCommit leading to wrong display of the history
in EGit. In such cases there is a never ending passing line drawn beside
all commits younger than the commit with duplicate parents. This commit
fixes this by explicitly checking for duplicate parents.

In a different commit we should fix JGit not to create commits with
duplicate parents. I think native git also doesn't allow such commits,
although history display in native git (gitk, git log --graph) is not
damaged by such commits.

Change-Id: Ie3019ef613a507023958bea27b1badc3b8950279

org.eclipse.jgit.test/tst/org/eclipse/jgit/revplot/PlotCommitListTest.java

@@ -75,6 +75,13 @@ public class PlotCommitListTest extends RevWalkTestCase {
 			return this;
 		}
 
+		public CommitListAssert nrOfPassingLanes(int lanes) {
+			assertEquals("Number of passing lanes of commit #"
+					+ (nextIndex - 1)
+					+ " not as expected.", lanes, current.passingLanes.length);
+			return this;
+		}
+
 		public CommitListAssert parents(RevCommit... parents) {
 			assertEquals("Number of parents of commit #" + (nextIndex - 1)
 					+ " not as expected.", parents.length,
@@ -308,4 +315,31 @@ public class PlotCommitListTest extends RevWalkTestCase {
 		test.commit(merge_fix).parents().lanePos(3);
 		test.noMoreCommits();
 	}
+
+	// test a history where a merge commit has two time the same parent
+	@Test
+	public void testDuplicateParents() throws Exception {
+		final RevCommit m1 = commit();
+		final RevCommit m2 = commit(m1);
+		final RevCommit m3 = commit(m2, m2);
+
+		final RevCommit s1 = commit(m2);
+		final RevCommit s2 = commit(s1);
+
+		PlotWalk pw = new PlotWalk(db);
+		pw.markStart(pw.lookupCommit(m3));
+		pw.markStart(pw.lookupCommit(s2));
+		PlotCommitList<PlotLane> pcl = new PlotCommitList<PlotLane>();
+		pcl.source(pw);
+		pcl.fillTo(Integer.MAX_VALUE);
+
+		CommitListAssert test = new CommitListAssert(pcl);
+		test.commit(s2).nrOfPassingLanes(0);
+		test.commit(s1).nrOfPassingLanes(0);
+		test.commit(m3).nrOfPassingLanes(1);
+		test.commit(m2).nrOfPassingLanes(0);
+		test.commit(m1).nrOfPassingLanes(0);
+		test.noMoreCommits();
+	}
+
 }

org.eclipse.jgit/src/org/eclipse/jgit/revplot/PlotCommit.java

@@ -100,9 +100,13 @@ public class PlotCommit<L extends PlotLane> extends RevCommit {
 		final int cnt = children.length;
 		if (cnt == 0)
 			children = new PlotCommit[] { c };
-		else if (cnt == 1)
-			children = new PlotCommit[] { children[0], c };
-		else {
+		else if (cnt == 1) {
+			if (!c.getId().equals(children[0].getId()))
+				children = new PlotCommit[] { children[0], c };
+		} else {
+			for (PlotCommit pc : children)
+				if (c.getId().equals(pc.getId()))
+					return;
 			final PlotCommit[] n = new PlotCommit[cnt + 1];
 			System.arraycopy(children, 0, n, 0, cnt);
 			n[cnt] = c;