aportupgrade: add support for specifying git ref

This adds support for specifying an arbitrary git ref (e.g. commits,
tags, branches) to upgrade to. This can be useful if a specific commit
needs to be packaged instead of the latest available. Alternatively you
can also specify a branch to be used if the default branch is 'stable'
but 'develop' should be packaged.

This also removes old code to use the 'bionic' branch for UBports Lomiri
(formerly Unity 8) packages.