erp5_forge: pre-fill changelog field

To save a bit of time every day (and save us from typos)
7 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed ERP5.CodingStyleTest-Master

00:44:00

failed ERP5.PerformanceTest-Master

00:32:50

failed ERP5.UnitTest-Master

01:24:05

passed SlapOS.Eggs.UnitTest-Master.Python2

00:16:19

passed SlapOS.Eggs.UnitTest-Master.Python3

00:14:58

failed SlapOS.Eggs.UnitTest-luke.Python2

00:14:21

passed SlapOS.Eggs.UnitTest-luke.Python3

00:15:06