Reduce the number of connects when fetching

Reduce the number of connects when fetching

This shares the connection between getting the remote ref list and
getting objects in the first batch. (A second connection is still used
to follow tags).

When we do not fetch objects (i.e. either ls-remote disconnects after
getting list of refs, or we decide we are already up-to-date), we
clean up the connection properly; otherwise the connection is left
open in need of cleaning up to avoid getting an error message from
the remote end when ssh is used.

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>