Update URL of Source Map (#1428)

Per #1427.

This also fixes the build-diff logic which duplicated the entry in the new
index it creates for testing purpose, because the check on whether specs are
effectively distinct did not take the spec's shortname into account.

specs.json

@@ -464,7 +464,7 @@
   },
   "https://tc39.es/proposal-temporal/",
   {
-    "url": "https://tc39.es/source-map-spec/",
+    "url": "https://tc39.es/source-map/",
     "shortname": "sourcemap",
     "nightly": {
       "status": "Editor's Draft",

src/build-diff.js

@@ -76,7 +76,7 @@ function areDistinctSpecsInSameSeries(s1, s2) {
     const computed2 = computeShortname(spec2.shortname ?? spec2.url, spec2.forkOf);
     const series1 = spec1?.series?.shortname ?? computed1.series.shortname;
     const series2 = spec2?.series?.shortname ?? computed2.series.shortname;
-    return series1 === series2;
+    return (computed1.shortname !== computed2.shortname) && (series1 === series2);
   }
   catch {
     return false;