git-clone: fetch possibly detached HEAD over dumb http

git-clone: fetch possibly detached HEAD over dumb http

git-clone supports cloning from a repo with detached HEAD,
but if this HEAD is not behind any branch tip then it
would not have been fetched over dumb http, resulting in a

	fatal: Not a valid object name HEAD

Since 928c210a, this would also happen on a http repo
with a HEAD that is a symbolic link where someone has
forgotton to run update-server-info.

Signed-off-by: Sven Verdoolaege <skimo@liacs.nl>
Signed-off-by: Junio C Hamano <gitster@pobox.com>