http-push: refactor request url creation

http-push: refactor request url creation

Introduce two helper functions append_remote_object_url() and
get_remote_object_url() and use them to remove various places
that allocate and format the URL by hand.  These functions generate
a URL that point at the fan-out directory inside the remote object
store (e.g. http://host/path/to/repo/objects/a1/) or at an individual
loose object file.

Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>