revisions API: clear "boundary_commits" in release_revisions()

revisions API: clear "boundary_commits" in release_revisions()

Clear the "boundary_commits" object_array in release_revisions(). This
makes a few more tests pass under SANITIZE=leak, including
"t/t4126-apply-empty.sh" which started failed as an UNLEAK() in
cmd_format_patch() was removed in a preceding commit.

This also re-marks the various tests relying on "git format-patch" as
passing under "SANITIZE=leak", in the preceding "revisions API users:
use release_revisions() in builtin/log.c" commit those were marked as
failing as we removed the UNLEAK(rev) from cmd_format_patch() in
"builtin/log.c".

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