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/.

build/variables.py

@@ -95,7 +95,7 @@ def source_repo_header(output):
         output.write("#define MOZ_SOURCE_STAMP %s\n" % changeset)
 
     if repo and buildconfig.substs.get("MOZ_INCLUDE_SOURCE_INFO"):
-        source = "%s/rev/%s" % (repo, changeset)
+        source = "%s/-/commit/%s" % (repo, changeset)
         output.write("#define MOZ_SOURCE_REPO %s\n" % repo)
         output.write("#define MOZ_SOURCE_URL %s\n" % source)