switch: reject "do nothing" case

"git checkout" can be executed without any arguments. What it does is
not exactly great: it switches from HEAD to HEAD and shows worktree
modification as a side effect.

Make switch reject this case. Just use "git status" if you want
that side effect. For switch, you have to either

- really switch a branch
- (explicitly) detach from the current branch
- create a new branch

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -55,6 +55,7 @@ struct checkout_opts {
 	int no_dwim_new_local_branch;
 	int discard_changes;
 	int accept_pathspec;
+	int switch_branch_doing_nothing_is_ok;
 
 	/*
 	 * If new checkout options are added, skip_merge_working_tree
@@ -1338,6 +1339,12 @@ static int checkout_branch(struct checkout_opts *opts,
 		die(_("Cannot switch branch to a non-commit '%s'"),
 		    new_branch_info->name);
 
+	if (!opts->switch_branch_doing_nothing_is_ok &&
+	    !new_branch_info->name &&
+	    !opts->new_branch &&
+	    !opts->force_detach)
+		die(_("missing branch or commit argument"));
+
 	if (new_branch_info->path && !opts->force_detach && !opts->new_branch &&
 	    !opts->ignore_other_worktrees) {
 		int flag;
@@ -1593,6 +1600,7 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 
 	memset(&opts, 0, sizeof(opts));
 	opts.no_dwim_new_local_branch = 0;
+	opts.switch_branch_doing_nothing_is_ok = 1;
 	opts.accept_pathspec = 1;
 
 	options = parse_options_dup(checkout_options);
@@ -1624,6 +1632,7 @@ int cmd_switch(int argc, const char **argv, const char *prefix)
 	memset(&opts, 0, sizeof(opts));
 	opts.no_dwim_new_local_branch = 0;
 	opts.accept_pathspec = 0;
+	opts.switch_branch_doing_nothing_is_ok = 0;
 
 	options = parse_options_dup(switch_options);
 	options = add_common_options(&opts, options);