Since 6d28644 (git-am: do not allow empty commits by mistake.,
2006-02-23), git-am --resolved will check to see if the index has any
changes to prevent the user from creating an empty commit by mistake.
Add a test for this.
Reviewed-by: Stefan Beller <sbeller@google.com> Reviewed-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Paul Tan <pyokagan@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>