apertis-pkg-merge-updates: fast-forward the main debian branch after a Debian release point

Updates available in -security and -proposed-updates fall into the main Debian repository
after a Debian release point. In case an update has been imported from -security or -proposed-updates,
our main Debian branch stays out of date. Consequently, the Dashboard wrongly complains about an
available update from Debian.

In this scenario, the main Debian branch needs to be synchronized with the
-security or -proposed-updates branch used previously.

Signed-off-by: Dylan Aïssi <dylan.aissi@collabora.com>