The intention was always that files only get written to *.partial
files and renamed at the end once fully received, but somewhere in the
process that got lost in buffered mode and *.partial files were only
being used in direct receive mode. This fix prevents WaitingFiles
from returning files that are still being transferred.
Updates tailscale/corp#1626
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>