git-clone.sh: properly configure remote even if remote's head is dangling

git-clone.sh: properly configure remote even if remote's head is dangling

When cloning a remote repository which's HEAD refers to a nonexistent
ref, git-clone cloned all existing refs, but failed to write the
configuration for 'remote'.  Now it detects the dangling remote HEAD,
refuses to checkout any local branch since HEAD refers to nowhere, but
properly writes the configuration for 'remote', so that subsequent
'git fetch's don't fail.

The problem was reported by Daniel Jacobowitz through
 http://bugs.debian.org/466581

Signed-off-by: Gerrit Pape <pape@smarden.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>