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.
While at it, also remove unnecessary "#!/usr/bin/env python3" in files
that only get imported, and adjust other empty/comment lines in the
beginnings of the files for consistency.
This makes files easier to read, and makes the pmbootstrap codebase more
consistent with the build.postmarketos.org codebase.
The gist of this action is upgrading the specified aport to the latest
version. There are implementations for both stable packages (which check
via the release-monitoring.org API for new versions) and git packages
(which check the GitLab/GitHub API for new commits on the main branch).
There's also the possibility to pass --all, --all-stable & --all-git to
the action which either loops through all packages, or just stable or
git packages and upgrades them.
The --dry argument is also respected.
Note, that the implementation does update the variables pkgver, pkgrel
and _commit but it doesn't update the checksums because that would slow
down the process a lot, and is potentially undesirable.
