fsmonitor: refactor bit invalidation in refresh callback

fsmonitor: refactor bit invalidation in refresh callback

Refactor code in the fsmonitor_refresh_callback() call chain dealing
with invalidating the CE_FSMONITOR_VALID bit and add a trace message.

During the refresh, we clear the CE_FSMONITOR_VALID bit in response to
data from the FSMonitor daemon (so that a later phase will lstat() and
verify the true state of the file).

Create a new function to clear the bit and add some unique tracing for
it to help debug edge cases.

This is similar to the existing `mark_fsmonitor_invalid()` function,
but it also does untracked-cache invalidation and we've already
handled that in the refresh-callback handlers, so but we don't need
to repeat that.

Signed-off-by: Jeff Hostetler <jeffhostetler@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>