The git_http_client.php example uses a http RepoURL (http://github.com/crodas/php-git.git) that returns a 301 response to redirect to the https URL which is not handled by wgetFile (GitHub redirects http requests to https).
Alright!, you found a bug, please report it Page not found http://github.com/crodas/php-git.git/HEAD
HTTP Request and Response:
GET /crodas/php-git.git/HEAD HTTP/1.1
HTTP/1.1 301 Moved Permanently
Date: Mon, 09 May 2011 21:19:07 GMT
Checking now, perhaps tomorrow I'll upload a patch.
I've sent a pull request with the patch: #5