SystemReader#now: make it a concrete method

Abstract methods break subclasses (e.g. DelegateSystemReader in
gerrit). Updating jgit and gerrit is simpler if we do not add them. I
am not sure why some methods are abstract and others dont, but now()
can be a concrete method.

Make now() concrete. Implement it by default based on
getCurrentTime(), so subclasses overriding that method get the same
value.

Change-Id: I697749f8cba698c5388ed13ebdc2b238d6259358

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

@@ -18,7 +18,6 @@
 import java.text.DateFormat;
 import java.text.SimpleDateFormat;
 import java.time.Duration;
-import java.time.Instant;
 import java.time.ZoneId;
 import java.time.ZoneOffset;
 import java.util.HashMap;
@@ -204,11 +203,6 @@ public long getCurrentTime() {
 		return now;
 	}
 
-	@Override
-	public Instant now() {
-		return Instant.ofEpochMilli(now);
-	}
-
 	@Override
 	public MonotonicClock getClock() {
 		return () -> {

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

@@ -532,7 +532,11 @@ private void updateAll(Config config)
 	 *
 	 * @since 7.1
 	 */
-	public abstract Instant now();
+	public Instant now() {
+		// Subclasses overriding getCurrentTime should keep working
+		// TODO(ifrade): Once we remove getCurrentTime, use Instant.now()
+		return Instant.ofEpochMilli(getCurrentTime());
+	}
 
 	/**
 	 * Get clock instance preferred by this system.