fsevent: coalesce watches nested inside other watches

[daandemeyer]

An FSEvents stream root reports the entire hierarchy below it, and
callback_impl attributes every event to the longest watch it falls
under, so passing both a directory and something below it to
FSEventStreamCreate adds nothing while costing a slot against the path
limit that macOS enforces by corrupting the process descriptor table
above roughly RLIMIT_NOFILE/10 paths.

Derive the CFArray handed to FSEventStreamCreate from the watch map in
run() instead of maintaining it alongside, dropping any watch whose
ancestors include another watch on the same volume. FSEvents does not
promise that a stream root reports events from a volume mounted below
it, so a covering ancestor across a mount point does not count.
Unwatching a covering directory promotes the nested watches back to
stream roots on the next run with no separate bookkeeping, and
remove_cf_path goes away.

One behavioural difference: with kFSEventStreamCreateFlagWatchRoot, a
nested watch no longer receives a root-changed event of its own when an
ancestor is renamed. A recursive covering watch still reports the
rename; a non-recursive one reports nothing, since callback_impl
accepts only direct children for those.

Covered by unit tests for the stream roots produced by nested and by
sibling watches, and by black-box tests that a nested watch still
delivers events after its covering watch is removed and that a covering
watch still delivers events outside the nested one.

Signed-off-by: Daan De Meyer daan@amutable.com

[JohnTitor]

Makes sense 👍 added a tweak around some coalescing condition.