Add lazy loading of body for tags to PlotWalk

This change is required by egit's lazy loading of the body in the
history view when the walk is started with setRetainBody(false).

Change-Id: I9291ba8c34c8744bc009b1bd302ed28bfa4e9476
Signed-off-by: Stefan Lay <stefan.lay@sap.com>

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

@@ -172,6 +172,11 @@ public class PlotWalk extends RevWalk {
 				return ((RevCommit) o).getCommitTime();
 			if (o instanceof RevTag) {
 				RevTag tag = (RevTag) o;
+				try {
+					parseBody(tag);
+				} catch (IOException e) {
+					return 0;
+				}
 				PersonIdent who = tag.getTaggerIdent();
 				return who != null ? who.getWhen().getTime() : 0;
 			}