gpick: save drop_closed from resolved insertion specs as well

an insertion spec may be matched up with a local series, in which case
the corresponding pushed series will need to be completed as well, thus
requiring a defined drop_closed value.

amends f3176160.

Change-Id: I1ab5c7579ef5491198158d8a2c3e569c3f9c3fb0
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>