Some committishes might point to the same commit, but through a
different ref, that's why it's better to check directly for the ref,
rather than the commit message.
We can do that by calling rev-parse --symbolic-full-name, and to
differentiate the old from the new behavior we add an extra argument to
the check() helper.
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>