Support automatic command line parsing for TextBuiltin subclasses

Hi Shawn

to use args4j was really a great idea. I had a patch, implementing something similar to args4j in my repository, but using a library is always better. *thumb up*

I had a short look at the files and noticed that the inner "if (help) {" is unnecessary:

+ if (help) {
+ System.err.print("jgit ");
+ System.err.print(commandName);
+ clp.printSingleLineUsage(System.err);
+ System.err.println();
+
+ if (help) {
+ System.err.println();
+ clp.printUsage(System.err);
+ System.err.println();
+ }
+ System.exit(1);
+ }

Also a cool function which you may want to use is:

public <U> Class<? extends U> asSubclass(Class<U> clazz)

With that method of Class<?> you can do the cast before you actually create the object.
You could for example make use of them in SubcommandHandler:
l. 124 cmd = (TextBuiltin) cons.newInstance();

Best regards,
Florian Köberle