PlotWalk: Replace call to deprecated method in PersonIdent

Change-Id: I0972121aaba6fd05f733d3ec62515e00c30530c7

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

diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/revplot/PlotWalk.java b/org.eclipse.jgit/src/org/eclipse/jgit/revplot/PlotWalk.java
index 0f6bd2d..39e8fd8 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/revplot/PlotWalk.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/revplot/PlotWalk.java
@@ -179,7 +179,7 @@ long timeof(RevObject o) {
 					return 0;
 				}
 				PersonIdent who = tag.getTaggerIdent();
-				return who != null ? who.getWhen().getTime() : 0;
+				return who != null ? who.getWhenAsInstant().toEpochMilli() : 0;
 			}
 			return 0;
 		}