t7501: test the right kind of breakage

t7501: test the right kind of breakage

These tests try to run "git commit" with various "forbidden" combinations
of options and expect the command to fail, but they do so without having
any change added to the index.  We wouldn't be able to catch breakages
that would allow these combinations by mistake with them because the
command will fail with "nothing to commit" anyway.

Make sure we have something added to the index before running the command.

Signed-off-by: Junio C Hamano <gitster@pobox.com>