built-in add -i: wire up the new C code for the `patch` command

The code in `git-add--interactive.perl` that takes care of the `patch`
command can look quite intimidating. There are so many modes in which it
can be called, for example.

But for the `patch` command in `git add -i`, only one mode is relevant:
the `stage` mode. And we just implemented the beginnings of that mode in
C so far. So let's use it when `add.interactive.useBuiltin=true`.

Now, while the code in `add-patch.c` is far from reaching feature parity
with the code in `git-add--interactive.perl` (color is not implemented,
the diff algorithm cannot be configured, the colored diff cannot be
post-processed via `interactive.diffFilter`, many commands are
unimplemented yet, etc), hooking it all up with the part of `git add -i`
that is already converted to C makes it easier to test and develop it.

Note: at this stage, both the `add.interactive.useBuiltin` config
setting is still safely opt-in, and will probably be fore quite some
time, to allow for thorough testing "in the wild" without adversely
affecting existing users.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

add-interactive.c

@@ -917,15 +917,18 @@ static int run_patch(struct add_i_state *s, const struct pathspec *ps,
 	count = list_and_choose(s, files, opts);
 	if (count >= 0) {
 		struct argv_array args = ARGV_ARRAY_INIT;
+		struct pathspec ps_selected = { 0 };
 
-		argv_array_pushl(&args, "git", "add--interactive", "--patch",
-				 "--", NULL);
 		for (i = 0; i < files->items.nr; i++)
 			if (files->selected[i])
 				argv_array_push(&args,
 						files->items.items[i].string);
-		res = run_command_v_opt(args.argv, 0);
+		parse_pathspec(&ps_selected,
+			       PATHSPEC_ALL_MAGIC & ~PATHSPEC_LITERAL,
+			       PATHSPEC_LITERAL_PATH, "", args.argv);
+		res = run_add_p(s->r, &ps_selected);
 		argv_array_clear(&args);
+		clear_pathspec(&ps_selected);
 	}
 
 	return res;