tests: at-combinations: improve nonsense()

tests: at-combinations: improve nonsense()

In some circumstances 'git log' might fail, but not because the @
parsing failed. For example: 'git rev-parse' might succeed and return a
bad object, and then 'git log' would fail.

The layer we want to test is revision parsing, so let's test that
directly.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>