test-lib: GIT_TEST_SANITIZE_LEAK_LOG enabled by default

test-lib: GIT_TEST_SANITIZE_LEAK_LOG enabled by default

As we currently describe in t/README, it can happen that:

    Some tests run "git" (or "test-tool" etc.) without properly checking
    the exit code, or git will invoke itself and fail to ferry the
    abort() exit code to the original caller.

Therefore, GIT_TEST_SANITIZE_LEAK_LOG=true is needed to be set to
capture all memory leaks triggered by our tests.

It seems unnecessary to force users to remember this option, as
forgetting it could lead to missed memory leaks.

We could solve the problem by making it "true" by default, but that
might suggest we think "false" makes sense, which isn't the case.

Therefore, the best approach is to remove the option entirely while
maintaining the capability to detect memory leaks in blind spots of our
tests.

Signed-off-by: Rubén Justo <rjusto@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>