sequencer: simplify allocation of result array in todo_list_rearrange_squash()

sequencer: simplify allocation of result array in todo_list_rearrange_squash()

The operation doesn't change the number of elements in the array, so we do
not need to allocate the result piecewise.

This moves the re-assignment of todo_list->alloc at the end slighly up,
so it's right after the newly added assert which also refers to `nr`
(and which indeed should come first). Also, the value is more likely to
be still in a register at that point.

Signed-off-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>