Merge branch 'cb/http-multi-curl-auth' into maint

HTTP transport that requires authentication did not work correctly when
multiple connections are used simultaneously.

By Jeff King (3) and Clemens Buchacher (1)
* cb/http-multi-curl-auth:
  http: use newer curl options for setting credentials
  http: clean up leak in init_curl_http_auth
  fix http auth with multiple curl handles
  http auth fails with multiple curl handles