gitweb: add `status` headers to git_feed() responses.

The git_feed() method was not setting a `Status` header unless it was
responding to an If-Modified-Since request with `304 Not Modified`.
Now, when it is serving successful responses, it sets status to `200
OK`.

Signed-off-by: W Trevor King <redacted>
Acked-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>