GitLab is used only for code review, issue tracking and project management. Canonical locations for source code are still https://gitweb.torproject.org/ https://git.torproject.org/ and git-rw.torproject.org.
Please install the diffoscope package on tb-build-02.
This is not needed for building, but I think it can be useful when we need to debug a reproducible build issue.
/cc @acat, @gk, @sysrqb