fix(git): limit ls-remote output to heads/tags (#97)

GitHub repositories can contain other types of refs, such as pull
requests, which can be fetched via `git-fetch` but not cloned via
`git clone -b`.

Fixes: npm/npm#16898

lib/util/git.js

@@ -105,7 +105,7 @@ function revs (repo, opts) {
     return BB.resolve(cached)
   }
   return pinflight(`ls-remote:${repo}`, () => {
-    return spawnGit(['ls-remote', repo], {
+    return spawnGit(['ls-remote', '-h', '-t', repo], {
       env: gitEnv()
     }, opts).then(child => {
       let stdout = ''