rev-parse test: use standard test functions for setup

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 <redacted>
Signed-off-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>