dir: introduce readdir_skip_dot_and_dotdot() helper

Many places in the code were doing
    while ((d = readdir(dir)) != NULL) {
        if (is_dot_or_dotdot(d->d_name))
            continue;
        ...process d...
    }
Introduce a readdir_skip_dot_and_dotdot() helper to make that a one-liner:
    while ((d = readdir_skip_dot_and_dotdot(dir)) != NULL) {
        ...process d...
    }

This helper particularly simplifies checks for empty directories.

Also use this helper in read_cached_dir() so that our statistics are
consistent across platforms.  (In other words, read_cached_dir() should
have been using is_dot_or_dotdot() and skipping such entries, but did
not and left it to treat_path() to detect and mark such entries as
path_none.)

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff-no-index.c

@@ -26,9 +26,8 @@ static int read_directory_contents(const char *path, struct string_list *list)
 	if (!(dir = opendir(path)))
 		return error("Could not open directory %s", path);
 
-	while ((e = readdir(dir)))
-		if (!is_dot_or_dotdot(e->d_name))
-			string_list_insert(list, e->d_name);
+	while ((e = readdir_skip_dot_and_dotdot(dir)))
+		string_list_insert(list, e->d_name);
 
 	closedir(dir);
 	return 0;