Before this change if a file was uploaded to a backend which didn't
support modtimes, the time of the file read after the upload had
completed would change to the time the file was uploaded on the
backend.
When using `--vfs-cache-mode writes` or `full` this time would be
different by the `--vfs-write-back` delay which would cause
applications to think the file had been modified.
This changes uses the last modification time read by the OS as a
virtual modtime for backends which don't support setting modtimes. It
does not change the modtime to that actually uploaded.
This means that as long as the file remains in the directory cache it
will have the expected modtime.
See: https://forum.rclone.org/t/saving-files-causes-wrong-modified-time-to-be-set-for-a-few-seconds-on-webdav-mount-with-bitrix24/36451
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