date API: add and use a date_mode_release()

Fix a memory leak in the parse_date_format() function by providing a
new date_mode_release() companion function.

By using this in "t/helper/test-date.c" we can mark the
"t0006-date.sh" test as passing when git is compiled with
SANITIZE=leak, and whitelist it to run under
"GIT_TEST_PASSING_SANITIZE_LEAK=true" by adding
"TEST_PASSES_SANITIZE_LEAK=true" to the test itself.

The other tests that expose this memory leak (i.e. take the
"mode->type == DATE_STRFTIME" branch in parse_date_format()) are
"t6300-for-each-ref.sh" and "t7004-tag.sh". The former is due to an
easily fixed leak in "ref-filter.c", and brings the failures in
"t6300-for-each-ref.sh" down from 51 to 48.

Fixing the remaining leaks will have to wait until there's a
release_revisions() in "revision.c", as they have to do with leaks via
"struct rev_info".

There is also a leak in "builtin/blame.c" due to its call to
parse_date_format() to parse the "blame.date" configuration. However
as it declares a file-level "static struct date_mode blame_date_mode"
to track the data, LSAN will not report it as a leak. It's possible to
get valgrind(1) to complain about it with e.g.:

    valgrind --leak-check=full --show-leak-kinds=all ./git -P -c blame.date=format:%Y blame README.md

But let's focus on things LSAN complains about, and are thus
observable with "TEST_PASSES_SANITIZE_LEAK=true". We should get to
fixing memory leaks in "builtin/blame.c", but as doing so would
require some re-arrangement of cmd_blame() let's leave it for some
other time.

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