built-in add -i: re-implement the diff command

It is not only laziness that we simply spawn `git diff -p --cached`
here: this command needs to use the pager, and the pager needs to exit
when the diff is done. Currently we do not have any way to make that
happen if we run the diff in-process. So let's just spawn.

Signed-off-by: Johannes Schindelin <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

add-interactive.c

@@ -931,6 +931,47 @@ static int run_patch(struct add_i_state *s, const struct pathspec *ps,
 	return res;
 }
 
+static int run_diff(struct add_i_state *s, const struct pathspec *ps,
+		    struct prefix_item_list *files,
+		    struct list_and_choose_options *opts)
+{
+	int res = 0;
+	ssize_t count, i;
+
+	struct object_id oid;
+	int is_initial = !resolve_ref_unsafe("HEAD", RESOLVE_REF_READING, &oid,
+					     NULL);
+	if (get_modified_files(s->r, INDEX_ONLY, files, ps, NULL, NULL) < 0)
+		return -1;
+
+	if (!files->items.nr) {
+		putchar('\n');
+		return 0;
+	}
+
+	opts->prompt = N_("Review diff");
+	opts->flags = IMMEDIATE;
+	count = list_and_choose(s, files, opts);
+	opts->flags = 0;
+	if (count >= 0) {
+		struct argv_array args = ARGV_ARRAY_INIT;
+
+		argv_array_pushl(&args, "git", "diff", "-p", "--cached",
+				 oid_to_hex(!is_initial ? &oid :
+					    s->r->hash_algo->empty_tree),
+				 "--", 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);
+		argv_array_clear(&args);
+	}
+
+	putchar('\n');
+	return res;
+}
+
 static int run_help(struct add_i_state *s, const struct pathspec *unused_ps,
 		    struct prefix_item_list *unused_files,
 		    struct list_and_choose_options *unused_opts)
@@ -1029,6 +1070,7 @@ int run_add_i(struct repository *r, const struct pathspec *ps)
 		{ "revert", run_revert },
 		{ "add untracked", run_add_untracked },
 		{ "patch", run_patch },
+		{ "diff", run_diff },
 		{ "help", run_help },
 	};
 	struct prefix_item_list commands = PREFIX_ITEM_LIST_INIT;