tests: change some 'test $(git) = "x"' to test_cmp

tests: change some 'test $(git) = "x"' to test_cmp

Change some of the patterns in the test suite where we were hiding the
exit code from "git" by invoking it in a sub-shell within a "test"
expression to use temporary files and test_cmp instead.

These are not all the occurrences of this anti-pattern, but these in
particular hid issues where LSAN was dying, and I'd thus marked these
tests as passing under the linux-leaks CI job in past commits with
"TEST_PASSES_SANITIZE_LEAK=true". Let's deal with that by either
removing that marking, or skipping specific tests under
!SANITIZE_LEAK.

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