CI: support release channels (v20.05 branch)

CI: common.py: add cache

Don't query the pmaports dir more than once. This cache will also be
used to store the upstream branch in the next commit.

CI: support release channels (v20.05 branch)

Add common.get_upstream_branch() and use it instead of assuming "master"
as upstream branch.

Example: user forks v20.05 to v20.05_some-fix, then creates a merge
request. CI must use v20.05 as upstream branch, when comparing for
modified files etc.

I've verified that it works on a v20.05 based branch: 8b93b630

Cherry-pick to v20.05 branch after merge (that's the point 😉).

Closes: #521 (closed)