index: support `git_index_add_all` directories with force

When the contents of an entire new directory is ignored, and `FORCE` is
specified to `git_index_add_all`, ensure that we expand the entire file
list. By default, diff will coalesce a fully ignored folder into a
single diff entry; expand it.

src/libgit2/index.c

@@ -3509,7 +3509,8 @@ static int index_apply_to_wd_diff(git_index *index, int action, const git_strarr
 			GIT_DIFF_RECURSE_UNTRACKED_DIRS;
 
 		if (flags == GIT_INDEX_ADD_FORCE)
-			opts.flags |= GIT_DIFF_INCLUDE_IGNORED;
+			opts.flags |= GIT_DIFF_INCLUDE_IGNORED |
+			              GIT_DIFF_RECURSE_IGNORED_DIRS;
 	}
 
 	if ((error = git_diff_index_to_workdir(&diff, repo, index, &opts)) < 0)