Fix history rendering not to occupy too many lanes

There was a bug in history rendering which caused jgit to use too
many lanes in case lanes get repositioned. Looking at commit
90c11cbaeb in JGit was one example. Vadim
Dmitriev found the problem and the solution.

Bug: 365460
Change-Id: I6024265b7a593dcfd4fc612d0baf6652a0092ff4
Also-by: Vadim Dmitriev <dmgloss@mail.ru>
Signed-off-by: Christian Halstrick <christian.halstrick@sap.com>

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

@@ -232,7 +232,9 @@ public class PlotCommitList<L extends PlotLane> extends
 			if (newPos == -1)
 				newPos = positionsAllocated++;
 			freePositions.add(commit.lane.getPosition());
+			activeLanes.remove(commit.lane);
 			commit.lane.position = newPos;
+			activeLanes.add(commit.lane);
 		}
 	}