Since d1c5f2a (Add git-am, applymbox replacement., 2005-10-07), git-am
will ensure that the index is clean before applying the patch. This is
to prevent changes unrelated to the patch from being committed.
Add a test for this check.
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>