When the release pull request was squashed on GitLab, the release
creation fails with error
Error: failed to create pending releases: pull request is missing the merge commit
This fixes the problem by checking both `MergeCommitSHA` and
`SquashCommitSHA` and using whichever is set.
Turns out that all we need is the path, and not the project id. The path
is way more user friendly, and we can easily get it from a CI variable
or combine it from the namespace & project name.
The GitLab project ID was still hardcoded to my playground project on
GitLab.com.
This commit instead reads from the predefined GitLab CI/CD variable for
the projects ID (`CI_PROJECT_ID`).
Makes navigating to the PR way easier when looking at the logs.
This requires a new `Forge.PullRequestURL()` method that needs to be
implemented for all forges.
