SystemReader: Respect passed-in parent when no system config is present

The default implementation of openSystemConfig has special handling for
when the FS returns null from getGitSystemConfig: it should return a
"real" FileBasedConfig instance that never actually tries to load a
file. However, this codepath was not respecting the passed-in parent
config.

Change-Id: Id0bcdc93bb42f9ebe3f5ee4c6b4be8863e0133f9

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

@@ -101,7 +101,7 @@ public abstract class SystemReader {
 		public FileBasedConfig openSystemConfig(Config parent, FS fs) {
 			File configFile = fs.getGitSystemConfig();
 			if (configFile == null) {
-				return new FileBasedConfig(null, fs) {
+				return new FileBasedConfig(parent, null, fs) {
 					@Override
 					public void load() {
 						// empty, do not load