http-backend: Use http.getanyfile to disable dumb HTTP serving

http-backend: Use http.getanyfile to disable dumb HTTP serving

Some repository owners may wish to enable smart HTTP, but disallow
dumb content serving.  Disallowing dumb serving might be because
the owners want to rely upon reachability to control which objects
clients may access from the repository, or they just want to
encourage clients to use the more bandwidth efficient transport.

If http.getanyfile is set to false the backend CGI will return with
'403 Forbidden' when an object file is accessed by a dumb client.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>