CI: changelog: fetch our repository

The changelog job requires to fetch the latest state of the repository, otherwise the result of this check might not be correct.

Up until now, 'origin' was fetched, which points, by default, to the remote repository a local checkout was cloned from, initially.

This works for our repository, but might fail for forks, depending on when they were forked and if they were updated in the meantime.

This commit ensures the correct repository is fetched. Besides this, the amount of data fetched is limited.

Ref !374 (closed)