[PATCH] Support alternates and http-alternates in http-fetch

[PATCH] Support alternates and http-alternates in http-fetch

This allows the remote repository to refer to additional repositories
in a file objects/info/http-alternates or
objects/info/alternates. Each line may be:

 a relative path, starting with ../, to get from the objects directory
  of the starting repository to the objects directory of the added
  repository.

 an absolute path of the objects directory of the added repository (on
  the same server).

 (only in http-alternates) a full URL of the objects directory of the
  added repository.

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>