undercover gem 0.6+ made filtering files configurable.
Their defaults cover common paths like `spec/`, `*_spec.rb` which is
missing paths like `ee/`, `jh/`, `qa/`, `gem/`, `vendor/`.

- ee/lib/**
- rubocop/**
- scripts/**
- sidekiq_cluster/**
- tooling/**

Otherwise undercoverage failures will not fail the CI job

We handle getting the merge base ourselves

See https://github.com/libgit2/rugged/issues/846

Also print key information for future debugging

This also tries to maintain running script/undercoverage without any
arguments.