git checkout: don't warn about unborn branch if -f is already passed

I think it's unnecessary to warn that the checkout has been forced due to an
unborn current branch if -f has been explicitly passed.  For one project, I am
using git-new-workdir to create workdirs from a bare repository whose HEAD is
set to an unborn branch, and this warning started to irritate me.

Signed-off-by: Matt McCutchen <matt@mattmccutchen.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-checkout.c

@@ -421,7 +421,7 @@ static int switch_branches(struct checkout_opts *opts, struct branch_info *new)
 	if (!opts->quiet && !old.path && old.commit && new->commit != old.commit)
 		describe_detached_head("Previous HEAD position was", old.commit);
 
-	if (!old.commit) {
+	if (!old.commit && !opts->force) {
 		if (!opts->quiet) {
 			fprintf(stderr, "warning: You appear to be on a branch yet to be born.\n");
 			fprintf(stderr, "warning: Forcing checkout of %s.\n", new->name);