Wrong computation of delta to find shortages

On the first loop, get_shortage adds a second times the delta of moves at
current date.

issue4215
review9751002
(grafted from f7020662a64c4a33b6ea1ad3bdf68656c4fd6339)

--HG--
branch : 2.8
extra : source : f7020662a64c4a33b6ea1ad3bdf68656c4fd6339