Remove unnecessary 'fetch' argument from transport_get API

Remove unnecessary 'fetch' argument from transport_get API

We don't actually need to know at the time of transport_get if the
caller wants to fetch, push, or do both on the returned object.
It is easier to just delay the initialization of the HTTP walker
until we know we will need it by providing a CURL specific fetch
function in the curl_transport that makes sure the walker instance
is initialized before use.

Signed-off-by: Shawn O. Pearce <redacted>
Signed-off-by: Junio C Hamano <redacted>