When adding a mail notifier, the email address is required. This means you can't notify only the latest committers.
The email address field should be optional, at least not required when the 'latest committers' checkbox is enabled.
As discussed on the user list: http://markmail.org/message/oma2qfwn562bjns5
A workaround is to provide a non-existent email address in the form, and let it bounce or fail. Or, provide a valid email address and configure filters to delete the messages.