Perl readdir detects list context and can return an array
suitable for the grep op. From there, we can rely on
substr to remove the ".git" suffix and integerize the value
to save a few bytes before letting List::Util::max return
the value.
This is how we detect Xapian shards nowadays, too, and
we'll also use defined-or (//) to simplify the return
value there.
We'll also simplify InboxWritable->git_dir_latest,
remove some callers, and consider removing it entirely.