tests: don't lose exit status with "(cd ...; test <op> $(git ...))"

tests: don't lose exit status with "(cd ...; test <op> $(git ...))"

Rewrite tests that ran "git" inside command substitution and lost the
exit status of "git" so that we notice the failing "git".

Have them use modern patterns such as a "test_cmp" of the expected
outputs instead.

We'll fix more of these these in the subsequent commit, for now we're
only converting the cases where this loss of exit code was combined
with spawning a sub-shell.

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