When two requests raced to complete the same multipart upload, the first
caller to finish would remove the part files and upload directory. The
second caller, already past the initial existence check, would then fail
mid-flight with confusing errors such as ErrInvalidPart or an I/O error
when trying to open a part that no longer exists.
Fix this by atomically renaming the upload directory from <uploadID> to
<uploadID>.inprogress at the very start of CompleteMultipartUploadWithCopy,
before any part data is read. A concurrent caller will now find the
original directory absent and receive a clean NoSuchUpload error. A
deferred rename restores the original name if the complete does not
succeed, allowing the client to retry.
ListMultipartUploads is updated to skip any directories whose name ends
in .inprogress so in-flight completes do not appear as pending uploads.
13b3dc5267