`MOZ_SOURCE_URL` is created out of `MOZ_SOURCE_REPO` and `MOZ_SOURCE_CHANGESET`. But the code takes for granted that it refers to a Hg instance, so combines them as `$MOZ_SOURCE_REPO/rev/$MOZ_SOURCE_CHANGESET`. We could change it the equivalent for GitLab, i.e., `$MOZ_SOURCE_REPO/-/commit/$MOZ_SOURCE_CHANGESET` (or the tree: `$MOZ_SOURCE_REPO/-/tree/$MOZ_SOURCE_CHANGESET`).