Fix 'git remote show' regression on empty repository in 1.5.4

Fix 'git remote show' regression on empty repository in 1.5.4

Back in 18f7c51c we switched git-ls-remote/git-peek-remote to
use the transport backend, rather than do everything itself.

As part of that switch we started to produce a non-zero exit
status if no refs were received from the remote peer, which
happens when the remote peer has no commits pushed to it yet.
(E.g. "git --git-dir=foo.git init; git ls-remote foo.git")

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>