Pop the new schedule buffers
ledger-schedule-upcoming take argument.

One can forget to run ledger-schedule-upcoming for some time. Then there
is the risk that ledger-schedule-look-backward is too short. This will
open the possibility to use a different file, or to change the period
backward or forward for this kind of case.
Merge pull request #223 from vanicat/t/schedule-with-args

I should have done this a long time ago, but I got too lazy.  Thanks Remi!