clone: drop extra newline from warning message

We don't need to put a "\n" in calls to warning(), since it adds one itself (and the user sees an extra blank line). Drop it, and while we're here, drop the full-stop from the message, which goes against our guidelines. This bug dates all the way back to 8434c2f1af (Build in clone, 2008-04-27), but presumably nobody noticed because it's hard to trigger: you have to clone a repository whose HEAD is unborn, but which is not otherwise empty. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>