Make http-fetch request types more clear

Rename object request functions and data to make it more clear which type
of request is being processed - this is a response to the introduction of
slot callbacks and the definition of different types of requests such as
alternates_request.

Signed-off-by: Nick Hengeveld <nickh@reactrix.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>