builtin/merge.c: use fixed strings, not "strbuf", fix leak

builtin/merge.c: use fixed strings, not "strbuf", fix leak

Follow-up 465028e0e25 (merge: add missing strbuf_release(),
2021-10-07) and address the "msg" memory leak in this block. We could
free "&msg" before the "goto done" here, but even better is to avoid
allocating it in the first place.

By repeating the "Fast-forward" string here we can avoid using a
"struct strbuf" altogether.

Suggested-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>