[WIP] .gitlab-ci.yml: Fix a check for polkadot to work on forked repos v2 (#4078)

* 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