pgm: Flush stderr during shutdown

If an error occurs in command line option parsing the error message
may be stuck in a Java level buffer.  Flush the stream to ensure
everything is made available to the calling process or terminal.

Change-Id: I55b16e43aa96fc81c1197121032f5cdd901d412c

org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/Main.java

@@ -224,6 +224,8 @@ public class Main {
 		} finally {
 			if (cmd.outw != null)
 				cmd.outw.flush();
+			if (cmd.errw != null)
+				cmd.errw.flush();
 		}
 	}