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>

builtin/clone.c

@@ -672,7 +672,7 @@ static int checkout(int submodule_progress, int filter_submodules)
 	head = resolve_refdup("HEAD", RESOLVE_REF_READING, &oid, NULL);
 	if (!head) {
 		warning(_("remote HEAD refers to nonexistent ref, "
-			  "unable to checkout.\n"));
+			  "unable to checkout"));
 		return 0;
 	}
 	if (!strcmp(head, "HEAD")) {