When we expect a git command to notice and signal errors, we
carelessly wrote in our tests:
test_expect_success 'reject bogus request' '
do something &&
do something else &&
! git command
'
but a non-zero exit could come from the "git command" segfaulting.
A new helper function "tset_must_fail" is introduced and it is
meant to be used to make sure the command gracefully fails (iow,
dying and exiting with non zero status is counted as a failure
to "gracefully fail"). The above example should be written as:
test_expect_success 'reject bogus request' '
do something &&
do something else &&
test_must_fail git command
'