This script indicate the downstream merges are to v2019 and v2020dev0.
However these two are already released. Now we should move the merges
targeting to v2020pre instead.
So this commit changed the downstream target from
apertis/v2019:apertis:v2020dev0 to apertis/v2020pre. If we need to
have apertis/v2020 release later. We may append this into the script.
Signed-off-by: Andrew Lee (李健秋) <firstname.lastname@example.org>