Handle the case where http transport gets redirected during the
authorization request better.

* jk/http-auth-redirects:
  http.c: Spell the null pointer as NULL
  remote-curl: rewrite base url from info/refs redirects
  remote-curl: store url as a strbuf
  remote-curl: make refs_url a strbuf
  http: update base URLs when we see redirects
  http: provide effective url to callers
  http: hoist credential request out of handle_curl_result
  http: refactor options to http_get_*
  http_request: factor out curlinfo_strbuf
  http_get_file: style fixes