commit-graph: stop fill_oids_from_packs() progress on error and free()

commit-graph: stop fill_oids_from_packs() progress on error and free()

Fix a bug in fill_oids_from_packs(), we should always stop_progress(),
but did not do so if we returned an error here. This also plugs a
memory leak in those cases by releasing the two "struct strbuf"
variables the function uses.

While I'm at it stop hardcoding "-1" here and just use the return
value of error() instead, which happens to be "-1".

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