Update http-push functionality

Update http-push functionality

This brings http-push functionality more in line with the ssh/git version,
by borrowing bits from send-pack and rev-list to process refspecs and
revision history in more standard ways.  Also, the status of remote objects
is determined using PROPFIND requests for the object directory rather than
HEAD requests for each object - while it may be less efficient for small
numbers of objects, this approach is able to get the status of all remote
loose objects in a maximum of 256 requests.

Signed-off-by: Junio C Hamano <junkio@cox.net>