restore: add --worktree and --staged

'git checkout <tree-ish> <pathspec>' updates both index and
worktree. But updating the index when you want to restore worktree
files is non-intuitive. The index contains the data ready for the next
commit, and there's no indication that the user will want to commit
the restored versions.

'git restore' therefore by default only touches worktree. The user has
the option to update either the index with

    git restore --staged --source=<tree> <path>  (1)

or update both with

    git restore --staged --worktree --source=<tree> <path> (2)

PS. Orignally I wanted to make worktree update default and form (1)
would add index update while also updating the worktree, and the user
would need to do "--staged --no-worktree" to update index only. But it
looks really confusing that "--staged" option alone updates both. So
now form (2) is used for both, which reads much more obvious.

PPS. Yes form (1) overlaps with "git reset <rev> <path>". I don't know
if we can ever turn "git reset" back to "_always_ reset HEAD and
optionally do something else".

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

builtin/checkout.c

@@ -66,6 +66,8 @@ struct checkout_opts {
 	int can_switch_when_in_progress;
 	int orphan_from_empty_tree;
 	int empty_pathspec_ok;
+	int checkout_index;
+	int checkout_worktree;
 
 	const char *new_branch;
 	const char *new_branch_force;
@@ -397,6 +399,7 @@ static int checkout_paths(const struct checkout_opts *opts,
 	struct commit *head;
 	int errs = 0;
 	struct lock_file lock_file = LOCK_INIT;
+	int checkout_index;
 
 	trace2_cmd_mode(opts->patch_mode ? "patch" : "path");
 
@@ -422,9 +425,26 @@ static int checkout_paths(const struct checkout_opts *opts,
 		die(_("Cannot update paths and switch to branch '%s' at the same time."),
 		    opts->new_branch);
 
-	if (opts->patch_mode)
-		return run_add_interactive(revision, "--patch=checkout",
-					   &opts->pathspec);
+	if (!opts->checkout_worktree && !opts->checkout_index)
+		die(_("neither '%s' or '%s' is specified"),
+		    "--staged", "--worktree");
+
+	if (!opts->checkout_worktree && !opts->from_treeish)
+		die(_("'%s' must be used when '%s' is not specified"),
+		    "--worktree", "--source");
+
+	if (opts->patch_mode) {
+		const char *patch_mode;
+
+		if (opts->checkout_index && opts->checkout_worktree)
+			patch_mode = "--patch=checkout";
+		else if (opts->checkout_index && !opts->checkout_worktree)
+			patch_mode = "--patch=reset";
+		else
+			die(_("'%s' with only '%s' is not currently supported"),
+			    "--patch", "--worktree");
+		return run_add_interactive(revision, patch_mode, &opts->pathspec);
+	}
 
 	repo_hold_locked_index(the_repository, &lock_file, LOCK_DIE_ON_ERROR);
 	if (read_cache_preload(&opts->pathspec) < 0)
@@ -482,10 +502,30 @@ static int checkout_paths(const struct checkout_opts *opts,
 		return 1;
 
 	/* Now we are committed to check them out */
-	errs |= checkout_worktree(opts);
+	if (opts->checkout_worktree)
+		errs |= checkout_worktree(opts);
 
-	if (write_locked_index(&the_index, &lock_file, COMMIT_LOCK))
-		die(_("unable to write new index file"));
+	/*
+	 * Allow updating the index when checking out from the index.
+	 * This is to save new stat info.
+	 */
+	if (opts->checkout_worktree && !opts->checkout_index && !opts->source_tree)
+		checkout_index = 1;
+	else
+		checkout_index = opts->checkout_index;
+
+	if (checkout_index) {
+		if (write_locked_index(&the_index, &lock_file, COMMIT_LOCK))
+			die(_("unable to write new index file"));
+	} else {
+		/*
+		 * NEEDSWORK: if --worktree is not specified, we
+		 * should save stat info of checked out files in the
+		 * index to avoid the next (potentially costly)
+		 * refresh. But it's a bit tricker to do...
+		 */
+		rollback_lock_file(&lock_file);
+	}
 
 	read_ref_full("HEAD", 0, &rev, NULL);
 	head = lookup_commit_reference_gently(the_repository, &rev, 1);
@@ -1461,6 +1501,20 @@ static int checkout_main(int argc, const char **argv, const char *prefix,
 	if (opts->overlay_mode == 1 && opts->patch_mode)
 		die(_("-p and --overlay are mutually exclusive"));
 
+	if (opts->checkout_index >= 0 || opts->checkout_worktree >= 0) {
+		if (opts->checkout_index < 0)
+			opts->checkout_index = 0;
+		if (opts->checkout_worktree < 0)
+			opts->checkout_worktree = 0;
+	} else {
+		if (opts->checkout_index < 0)
+			opts->checkout_index = -opts->checkout_index - 1;
+		if (opts->checkout_worktree < 0)
+			opts->checkout_worktree = -opts->checkout_worktree - 1;
+	}
+	if (opts->checkout_index < 0 || opts->checkout_worktree < 0)
+		BUG("these flags should be non-negative by now");
+
 	/*
 	 * From here on, new_branch will contain the branch to be checked out,
 	 * and new_branch_force and new_orphan_branch will tell us which one of
@@ -1617,6 +1671,8 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 	opts.orphan_from_empty_tree = 0;
 	opts.empty_pathspec_ok = 1;
 	opts.overlay_mode = -1;
+	opts.checkout_index = -2;    /* default on */
+	opts.checkout_worktree = -2; /* default on */
 
 	options = parse_options_dup(checkout_options);
 	options = add_common_options(&opts, options);
@@ -1674,6 +1730,10 @@ int cmd_restore(int argc, const char **argv, const char *prefix)
 	struct option restore_options[] = {
 		OPT_STRING('s', "source", &opts.from_treeish, "<tree-ish>",
 			   N_("where the checkout from")),
+		OPT_BOOL('S', "staged", &opts.checkout_index,
+			   N_("restore the index")),
+		OPT_BOOL('W', "worktree", &opts.checkout_worktree,
+			   N_("restore the working tree (default)")),
 		OPT_BOOL(0, "overlay", &opts.overlay_mode, N_("use overlay mode")),
 		OPT_END()
 	};
@@ -1684,6 +1744,8 @@ int cmd_restore(int argc, const char **argv, const char *prefix)
 	opts.accept_pathspec = 1;
 	opts.empty_pathspec_ok = 0;
 	opts.overlay_mode = 0;
+	opts.checkout_index = -1;    /* default off */
+	opts.checkout_worktree = -2; /* default on */
 
 	options = parse_options_dup(restore_options);
 	options = add_common_options(&opts, options);