fetch-pack: move common check and marking together

fetch-pack: move common check and marking together

When receiving 'ACK <object-id> continue' for a common commit, check if
the commit was already known to be common and mark it as such if not up
front. This should make future refactoring of how the information about
common commits is stored more straightforward.

No visible change intended.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>