Attempt to fix graph layout when new heads are introduced

Sometime the new commit is no allocated onto a new lane leading to
the commit being drawn on the wrong branch and something that looks
like a merge.

The drawback is that this also changes existing valid graphs.

Bug: 368927
Change-Id: Ic8a8247c8a53be802c1be83850ed766b902ca646

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

@@ -156,8 +156,8 @@ public class PlotCommitListTest extends RevWalkTestCase {
 
 		CommitListAssert test = new CommitListAssert(pcl);
 		test.commit(c).lanePos(0).parents(a);
-		test.commit(b).lanePos(1).parents(a);
-		test.commit(a).lanePos(0).parents();
+		test.commit(b).lanePos(0).parents(a);
+		test.commit(a).lanePos(1).parents();
 		test.noMoreCommits();
 	}
 
@@ -179,9 +179,9 @@ public class PlotCommitListTest extends RevWalkTestCase {
 
 		CommitListAssert test = new CommitListAssert(pcl);
 		test.commit(d).lanePos(0).parents(a);
-		test.commit(c).lanePos(1).parents(a);
-		test.commit(b).lanePos(1).parents(a);
-		test.commit(a).lanePos(0).parents();
+		test.commit(c).lanePos(0).parents(a);
+		test.commit(b).lanePos(0).parents(a);
+		test.commit(a).lanePos(1).parents();
 		test.noMoreCommits();
 	}
 
@@ -214,14 +214,45 @@ public class PlotCommitListTest extends RevWalkTestCase {
 		CommitListAssert test = new CommitListAssert(pcl);
 		test.commit(g).lanePos(0).parents(f);
 		test.commit(f).lanePos(0).parents(a);
-		test.commit(e).lanePos(1).parents(a);
-		test.commit(d).lanePos(1).parents(a);
-		test.commit(c).lanePos(1).parents(a);
-		test.commit(b).lanePos(1).parents(a);
-		test.commit(a).lanePos(0).parents();
+		test.commit(e).lanePos(0).parents(a);
+		test.commit(d).lanePos(0).parents(a);
+		test.commit(c).lanePos(0).parents(a);
+		test.commit(b).lanePos(0).parents(a);
+		test.commit(a).lanePos(1).parents();
 		test.noMoreCommits();
 	}
 
+	@Test
+	public void testBug368927() throws Exception {
+		final RevCommit a = commit();
+		final RevCommit b = commit(a);
+		final RevCommit c = commit(b);
+		final RevCommit d = commit(b);
+		final RevCommit e = commit(c);
+		final RevCommit f = commit(e, d);
+		final RevCommit g = commit(a);
+		final RevCommit h = commit(f);
+		final RevCommit i = commit(h);
+
+		PlotWalk pw = new PlotWalk(db);
+		pw.markStart(pw.lookupCommit(i.getId()));
+		pw.markStart(pw.lookupCommit(g.getId()));
+
+		PlotCommitList<PlotLane> pcl = new PlotCommitList<PlotLane>();
+		pcl.source(pw);
+		pcl.fillTo(Integer.MAX_VALUE);
+		CommitListAssert test = new CommitListAssert(pcl);
+		test.commit(i).lanePos(1).parents(h);
+		test.commit(h).lanePos(1).parents(f);
+		test.commit(g).lanePos(0).parents(a);
+		test.commit(f).lanePos(1).parents(e, d);
+		test.commit(e).lanePos(0).parents(c);
+		test.commit(d).lanePos(1).parents(b);
+		test.commit(c).lanePos(0).parents(b);
+		test.commit(b).lanePos(1).parents(a);
+		test.commit(a).lanePos(2).parents();
+	}
+
 	// test the history of the egit project between 9fdaf3c1 and e76ad9170f
 	@Test
 	public void testEgitHistory() throws Exception {

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

@@ -119,8 +119,12 @@ public class PlotCommitList<L extends PlotLane> extends
 		setupChildren(currCommit);
 
 		final int nChildren = currCommit.getChildCount();
-		if (nChildren == 0)
+		if (nChildren == 0) {
+			currCommit.lane = nextFreeLane();
+			activeLanes.add(currCommit.lane);
+			closeLane(currCommit.lane);
 			return;
+		}
 
 		if (nChildren == 1 && currCommit.children[0].getParentCount() < 2) {
 			// Only one child, child has only us as their parent.