SystemReader: Give a default implementation to #getTimezoneAt()

This abstract method forces subclasses (e.g. DelegateSystemReader in
gerrit) to update their code, but there is no strong reason to make it
abstract (subclasses can override it if needed).

Make the method concrete using the current default implementation
(which is the same in the mock).

Change-Id: Id1df0d71cab1d86879adf48e782f0050d3abcfa9

org.eclipse.jgit.junit/src/org/eclipse/jgit/junit/MockSystemReader.java

@@ -244,11 +244,6 @@ public int getTimezone(long when) {
 		return getTimeZone().getOffset(when) / (60 * 1000);
 	}
 
-	@Override
-	public ZoneOffset getTimeZoneAt(Instant when) {
-		return getTimeZoneId().getRules().getOffset(when);
-	}
-
 	@Override
 	public TimeZone getTimeZone() {
 		return TimeZone.getTimeZone("GMT-03:30");

org.eclipse.jgit/src/org/eclipse/jgit/util/SystemReader.java

@@ -178,11 +178,6 @@ public Instant now() {
 		public int getTimezone(long when) {
 			return getTimeZone().getOffset(when) / (60 * 1000);
 		}
-
-		@Override
-		public ZoneOffset getTimeZoneAt(Instant when) {
-			return getTimeZoneId().getRules().getOffset(when);
-		}
 	}
 
 	/**
@@ -569,7 +564,9 @@ public MonotonicClock getClock() {
 	 * @return the local time zone
 	 * @since 7.1
 	 */
-	public abstract ZoneOffset getTimeZoneAt(Instant when);
+	public ZoneOffset getTimeZoneAt(Instant when) {
+		return getTimeZoneId().getRules().getOffset(when);
+	}
 
 	/**
 	 * Get system time zone, possibly mocked for testing