rev-parse test: use standard test functions for setup
Save the reader from learning specialized t6* setup functions
where familiar commands like test_commit, "git checkout --orphan",
and "git merge" will do.
While at it, wrap the setup commands in a test assertion so errors can
be caught and stray output suppressed when running without --verbose
as in other tests.
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>