mailmap tests: get rid of overly complex blame fuzzing

mailmap tests: get rid of overly complex blame fuzzing

Change a test that used a custom fuzzing function since
bfdfa3d414 (t4203 (mailmap): stop hardcoding commit ids and dates,
2010-10-15) to just use the "blame --porcelain" output instead.

We could use the same pattern as 0ba9c9a0fb (t8008: rely on
rev-parse'd HEAD instead of sha1 value, 2017-07-26) does to do this,
but there wouldn't be any point. We're not trying to test "blame"
output here in general, just that "blame" pays attention to the
mailmap.

So it's sufficient to get the blamed line(s) and authors from the
output, which is much easier with the "--porcelain" option.

It would still be possible for there to be a bug in "blame" such that
it uses the mailmap for its "--porcelain" output, but not the regular
output. Let's test for that simply by checking if specifying the
mailmap changes the output.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>