serve.c: move version line to advertise_capabilities()

serve.c: move version line to advertise_capabilities()

The advertise_capabilities() is only called from serve() and we always
emit this version line before it. In a subsequent commit I'll make
builtin/upload-pack.c sometimes call advertise_capabilities()
directly, so it'll make sense to have this line emitted by
advertise_capabilities(), not serve() itself.

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