fetch-pack: give up after getting too many "ack continue"

If your repository have more roots than the remote repository
you ask an object for, the remote upload-pack keeps responding
"ack continue" until it fills up its received-have buffer
(currently 256 entries).  Usually this is not a problem because
the requester stops traversing the ancestry chain from the commit
it gets "ack continue" for, but this mechanism does not work as
a roadblock when it traverses down the path to the root the
other side does not have.

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