Fix comparison of empty revision with last one

revision can be None and so can not be compared to a datetime
(grafted from cfbc1c13f76252f7c1ef1718c837e9e423ed29d4)

--HG--
branch : 3.2
extra : source : cfbc1c13f76252f7c1ef1718c837e9e423ed29d4
Clear entry when combobox is set to inactive item

GTK doesn't clear the entry when calling set_active with -1.

issue4298
review13691002
(grafted from 736b752de550dd76233e7a1b25b28c2ef1a02c48)

--HG--
branch : 3.2
extra : source : 736b752de550dd76233e7a1b25b28c2ef1a02c48