upload-pack: change allow_unadvertised_object_request to an enum

upload-pack: change allow_unadvertised_object_request to an enum

As we cleanup 'upload-pack.c' by using 'struct upload_pack_data'
more thoroughly, let's change allow_unadvertised_object_request,
which is now part of 'upload_pack_data', from an 'unsigned int'
to an enum.

This will make it clear which values this variable can take.

While at it let's change this variable name to 'allow_uor' to
make it shorter.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>