There might be a problem with this patch.  Directory entries that are
*symlinks* to other directories do not increase the containing
directory's link count, but we do count them as directories in
add_files pass 1 and traverse in to them.  Hence, if you had a
directory that contained no sub-directories, but did contain symlinks
to other directories, we would fail to notice changes in the symlinked
directories.

We could check if the database thinks there are sub-directories and
only bail early if the directory is unchanged and *both* the file
system and the database think there are no sub-directories.

Quoth myself on Oct 24 at  4:33 pm:
> This can substantially reduce the cost of notmuch new in some
> situations, such as when the file system cache is cold or when the
> Maildir is on NFS.
> ---
>  notmuch-new.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/notmuch-new.c b/notmuch-new.c
> index faa33f1..364c73a 100644
> --- a/notmuch-new.c
> +++ b/notmuch-new.c
> @@ -323,6 +323,26 @@ add_files (notmuch_database_t *notmuch,
>      }
>      db_mtime = directory ? notmuch_directory_get_mtime (directory) : 0;
>  
> +    /* If the directory is unchanged from our last scan and has no
> +     * sub-directories, then return without scanning it at all.  In
> +     * some situations, skipping the scan can substantially reduce the
> +     * cost of notmuch new, especially since the huge numbers of files
> +     * in Maildirs make scans expensive, but all files live in leaf
> +     * directories.
> +     *
> +     * To check for sub-directories, we borrow a trick from find,
> +     * kpathsea, and many other UNIX tools: since a directory's link
> +     * count is the number of sub-directories (specifically, their
> +     * '..' entries) plus 2 (the link from the parent and the link for
> +     * '.').  This check is safe even on weird file systems, since
> +     * file systems that can't compute this will return 0 or 1.  This
> +     * is safe even on *really* weird file systems like HFS+ that
> +     * mistakenly return the total number of directory entries, since
> +     * that only inflates the count beyond 2.
> +     */
> +    if (directory && fs_mtime == db_mtime && st.st_nlink == 2)
> +	goto DONE;
> +
>      /* If the database knows about this directory, then we sort based
>       * on strcmp to match the database sorting. Otherwise, we can do
>       * inode-based sorting for faster filesystem operation. */