branch: add a dry_run parameter to create_branch()

Add a dry_run parameter to create_branch() such that dry_run = 1 will
validate a new branch without trying to create it. This will be used in
`git branch --recurse-submodules` to ensure that the new branch can be
created in all submodules.

Signed-off-by: Glen Choo <chooglen@google.com>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -893,7 +893,8 @@ static void update_refs_for_switch(const struct checkout_opts *opts,
 				      opts->new_branch_force ? 1 : 0,
 				      opts->new_branch_log,
 				      opts->quiet,
-				      opts->track);
+				      opts->track,
+				      0);
 		new_branch_info->name = opts->new_branch;
 		setup_branch_path(new_branch_info);
 	}