checkout: split part of it to new command 'restore'

Previously the switching branch business of 'git checkout' becomes a
new command 'switch'. This adds the restore command for the checking
out paths path.

Similar to git-switch, a new man page is added to describe what the
command will become. The implementation will be updated shortly to
match the man page.

A couple main differences from 'git checkout <paths>':

- 'restore' by default will only update worktree. This matters more
  when --source is specified ('checkout <tree> <paths>' updates both
  worktree and index).

- 'restore --staged' can be used to restore the index. This command
  overlaps with 'git reset <paths>'.

- both worktree and index could also be restored at the same time
  (from a tree) when both --staged and --worktree are specified. This
  overlaps with 'git checkout <tree> <paths>'

- default source for restoring worktree and index is the index and
  HEAD respectively. A different (tree) source could be specified as
  with --source (*).

- when both index and worktree are restored, --source must be
  specified since the default source for these two individual targets
  are different (**)

- --no-overlay is enabled by default, if an entry is missing in the
  source, restoring means deleting the entry

(*) I originally went with --from instead of --source. I still think
  --from is a better name. The short option -f however is already
  taken by force. And I do think short option is good to have, e.g. to
  write -s@ or -s@^ instead of --source=HEAD.

(**) If you sit down and think about it, moving worktree's source from
  the index to HEAD makes sense, but nobody is really thinking it
  through when they type the commands.

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

builtin/checkout.c

@@ -38,6 +38,11 @@ static const char * const switch_branch_usage[] = {
 	NULL,
 };
 
+static const char * const restore_usage[] = {
+	N_("git restore [<options>] [<branch>] -- <file>..."),
+	NULL,
+};
+
 struct checkout_opts {
 	int patch_mode;
 	int quiet;
@@ -1622,3 +1627,24 @@ int cmd_switch(int argc, const char **argv, const char *prefix)
 	FREE_AND_NULL(options);
 	return ret;
 }
+
+int cmd_restore(int argc, const char **argv, const char *prefix)
+{
+	struct checkout_opts opts;
+	struct option *options = NULL;
+	int ret;
+
+	memset(&opts, 0, sizeof(opts));
+	opts.dwim_new_local_branch = 1;
+	opts.switch_branch_doing_nothing_is_ok = 0;
+	opts.accept_pathspec = 1;
+
+	options = parse_options_dup(options);
+	options = add_common_options(&opts, options);
+	options = add_checkout_path_options(&opts, options);
+
+	ret = checkout_main(argc, argv, prefix, &opts,
+			    options, restore_usage);
+	FREE_AND_NULL(options);
+	return ret;
+}