Sometimes, running a CI pipeline is a game of whack-a-mole with the various
configurations and different build targets. Let's add a ci-fairy command that
can be run after pushing to wait for the current pipeline(s) to finish.

Usage: ci-fairy wait-for-pipeline

There are a few optional arguments, but most of the time it should find the
information itself. It picks HEAD from cwd by default, figures out the repo
etc. and then monitors the running pipeline.

Note that the repo lookup is *not* using the git config because we cannot know
which repository is the intended one. We cannot use the current branch's
upstream either because there's a reasonable chance it's origin/master.

So we just assume that the pipeline after pushing runs in the user fork,
defined by $GITLAB_USER_ID (or as fallback $USER). Where this isn't
sufficient, specify --project.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>