merge: add missing strbuf_release()

We strbuf_reset() this "struct strbuf" in a loop earlier, but never
freed it. Plugs a memory leak that's been here ever since this code
got introduced in 1c7b76be7d6 (Build in merge, 2008-07-07).

This takes us from 68 failed tests in "t7600-merge.sh" to 59 under
SANITIZE=leak, and makes "t7604-merge-custom-message.sh" pass!

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