t6019: modernize tests with helper

The tests in t6019 are repetitive, so create a helper that greatly
simplifies the test script.

In addition, update the common pattern that places 'git rev-list' on the
left side of a pipe, which can hide some exit codes. Send the output to
a 'raw' file that is then consumed by other tools so the Git exit code
is verified as zero.  And since we're using --format anyway, switch to
`git log`, so that we get the desired format and can avoid using sed.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Acked-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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