bugreport tests: tighten up "git bugreport -s hooks" test

bugreport tests: tighten up "git bugreport -s hooks" test

Amend a test added in 788a776069b (bugreport: collect list of
populated hooks, 2020-05-07) to "test_cmp" for the expected output,
instead of selectively using "grep" to check for specific things we
either expect or don't expect in the output.

As noted in a preceding commit our .git/hooks directory already
contains *.sample hooks, so we have no need to clobber the
prepare-commit-msg.sample hook in particular.

Instead we should assert that those *.sample hooks are not included in
the output, and for good measure let's add a new "unknown-hook", to
check that we only look through our own known hooks. See
cfe853e66be (hook-list.h: add a generated list of hooks, like
config-list.h, 2021-09-26) for how we generate that data.

We're intentionally not piping the "actual" output through "sort" or
similar, we'd also like to check that our reported hooks are sorted.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>