Until now, mu would _not_ follow symlinks; with these changes, we do.
There were some complications with that ~10 years ago, but I forgot the
details. So let's re-enable. At least one thing is in place now: moving
between file systems.
Fixes#1489Fixes#1628 (technically, this came with slightly earlier commit)
Add an option --lazy-check to ignore any directories that don't have
their ctime changed since the last indexing operation.
There are a few corner-cases (such as editing a message outside mu's
control) where this might miss a change, but apart from that, makes
indexing in for a maildir (and its sub-maildirs) almost a no-op if there
were no changes.