notmuch-new: Fix notmuch new to look at files within symbolic links

We look at the modified time of the database and the directory
to decide whether we need to look at only the subdirectories.
ie, if directory modified time is < database modified time
then we have already looking at all the files withing the
directory. So we just need to iterate through the subdirectories

But with symlinks we need to make sure we follow them even if
the directory modified time is less than database modified time

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>