merge: setup `opts` later in `checkout_fast_forward()`

merge: setup `opts` later in `checkout_fast_forward()`

After we initialize the various fields in `opts` but before we actually
use them, we might return early. Move the initialization further down,
to immediately before we use `opts`.

This limits the scope of `opts` and will help a later commit fix a
memory leak without having to worry about those early returns.

This patch is best viewed using something like this (note the tab!):
--color-moved --anchored="	trees[nr_trees] = parse_tree_indirect"

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>