Release ArchiveCommand's ObjectReader in call()

Release ArchiveCommand's ObjectReader in call()

Make call() release all private resources so instead of using a
pattern like

	ArchiveCommand cmd = git.archive();
	try {
		cmd.setTree(tree)
			. ...
			.call();
	} finally {
		cmd.release();
	}

callers can just use git.archive().setTree(tree)....call() directly.

This involves pushing more work out of parameter setters and into
call() so the ObjectReader is not allocated and potentially leaked
before then.

Change-Id: I699f703c6302696e1cc276d7ab8ee597d82f2c5d