Merge branch 'ef/checkout-empty' into maint

Running "git checkout" on an unborn branch used to corrupt HEAD
(regression in 1.7.10); this makes it error out.

By Erik Faye-Lund
* ef/checkout-empty:
  checkout: do not corrupt HEAD on empty repo

builtin/checkout.c

@@ -1091,7 +1091,7 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 	if (opts.writeout_stage)
 		die(_("--ours/--theirs is incompatible with switching branches."));
 
-	if (!new.commit) {
+	if (!new.commit && opts.new_branch) {
 		unsigned char rev[20];
 		int flag;