* Fix a check for polkadot to work on forked repos.
* .gitlab-ci.yml: Add `git diff` for CI debugging
* More future proof and less redundant.
* .gitlab-ci.yml: Improve comments and refactor sed command
* .gitlab-ci.yml: Remove two commas in a row
* .gitlab-ci.yml: Ensure to match branch statements at the end
* .gitlab-ci.yml: Reference concrete commit
When one does not specify the concrete commit, cargo-update tries to
checkout 'master' in the Substrate repository.
* .gitlab-ci.yml: Remove 'git diff' debug line