Bug 41603: Customize the creation of MOZ_SOURCE_URL

MOZ_SOURCE_URL is created by combining MOZ_SOURCE_REPO and
MOZ_SOURCE_CHANGESET.
But the code takes for granted that it refers to a Hg instance, so it
combines them as `$MOZ_SOURCE_REPO/rev/$MOZ_SOURCE_CHANGESET`.
With this commit, we change this logic to combine them to create a URL
that is valid for GitLab.
$MOZ_SOURCE_CHANGESET needs to be a commit hash, not a branch or a tag.
If that is needed, we could use /-/tree/, instead of /-/commit/.