Customize the creation of MOZ_SOURCE_URL

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).