Wrong computation of delta to find shortage

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

issue4215
review6561002
(grafted from 9e6d4c48af65ed734ab945da89971497d8c81335)

--HG--
branch : 2.8
extra : source : 9e6d4c48af65ed734ab945da89971497d8c81335