swift: fix update multipart object removing all of its own parts
swift: fix update multipart object removing all of its own parts

After uploading a multipart object, rclone deletes any unused parts.

Probably as part of the listing unification, the detection of the
parts beloning to the current upload was failing and calling Update
was deleting the parts for the current object.

This change fixes the detection and deletes all the old parts but none
of the new ones now.

Fixes #4075

ncw/rclone,ncw/rclone,ncw/rclone,ncw/rclone