sequencer: fix a memory leak in do_reset()

sequencer: fix a memory leak in do_reset()

Fix a memory leak introduced in 9055e401dd6 (sequencer: introduce new
commands to reset the revision, 2018-04-25), which called
setup_unpack_trees_porcelain() without a corresponding call to
clear_unpack_trees_porcelain().

This introduces a change in behavior in that we now start calling
clear_unpack_trees_porcelain() even without having called the
setup_unpack_trees_porcelain(). That's OK, that clear function, like
most others, will accept a zero'd out struct.

This inches us closer to passing various tests in
"t34*.sh" (e.g. "t3434-rebase-i18n.sh"), but because they have so many
other memory leaks in revisions.c this doesn't make any test file or
even a single test pass.

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