Prepare to base postmarketOS on Alpine stable by parsing the new
channels.cfg file in pmaports.git, that describes which channel
needs which branches and mirror dirs from postmarketOS and Alpine.
Use the information in pmb.helpers.git.get_branches_official() first,
more is coming in follow-up commits.
Read the file from origin/master, so we get the latest fetched version
even if the last checked out master branch is not up-to-date (think of
currently checked out release branch instead of master, master will
never be updated to point to latest origin/master). Allow to override
the file with a new --config-channels parameter.
Related: https://postmarketos.org/channels.cfg
c616874443