clone: disconnect transport after fetching

clone: disconnect transport after fetching

The current code just leaves the transport in whatever state
it was in after performing the fetch.  For a non-empty clone
over the git protocol, the transport code already
disconnects at the end of the fetch.

But for an empty clone, we leave the connection hanging, and
eventually close the socket when clone exits. This causes
the remote upload-pack to complain "the remote end hung up
unexpectedly". While this message is harmless to the clone
itself, it is unnecessarily scary for a user to see and may
pollute git-daemon logs.

This patch just explicitly calls disconnect after we are
done with the remote end, which sends a flush packet to
upload-pack and cleanly disconnects, avoiding the error
message.

Other transports are unaffected or slightly improved:

 - for a non-empty repo over the git protocol, the second
   disconnect is a no-op (since we are no longer connected)

 - for "walker" transports (like HTTP or FTP), we actually
   free some used memory (which previously just sat until
   the clone process exits)

 - for "rsync", disconnect is always a no-op anyway

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