Fix isRootRelative for protocol-relative URLs

Protocol-relative URLs, e.g., `//foo.test`, are not root-relative. This
fixes `isRootRelative` to return `false` when given a protocol-relative
URL using a negative lookahead assertion for a second '/' character.

app/assets/javascripts/lib/utils/url_utility.js

@@ -323,7 +323,7 @@ export function isAbsolute(url) {
  * @param {String} url
  */
 export function isRootRelative(url) {
-  return /^\//.test(url);
+  return /^\/(?!\/)/.test(url);
 }
 
 /**

spec/frontend/lib/utils/url_utility_spec.js

@@ -471,6 +471,7 @@ describe('URL utility', () => {
       ${'notaurl'}                              | ${false}
       ${'../relative_url'}                      | ${false}
       ${'<a></a>'}                              | ${false}
+      ${'//other-host.test'}                    | ${false}
     `('returns $valid for $url', ({ url, valid }) => {
       expect(urlUtils.isRootRelative(url)).toBe(valid);
     });