Write rclone uploads under a temporary name and move them into place (closes #266)

The rclone backend wrote each object straight to its key, so killing an
upload to a local or sftp remote left a truncated object there that the
next backup trusted.

On every remote with a server-side move, an object is now written under
a name ending in `.partial` and moved onto its key with rclone's
operations.Move, which removes an object already at the key first;
drive, dropbox and others will not move onto one. Listings skip
`.partial` names. Rclone's own copy also requires the PartialUploads
flag; this does not, because hdfs shows a file while it is written
without setting it. Remotes without a move are written in place.

Model: opus-5-5

internal/storage/file.go

@@ -50,9 +50,10 @@ const storageDirPerm = 0o755
 // temp file carrying this suffix and only renames it onto the real key once
 // the whole object is on disk, so an interrupted write can never leave a
 // truncated object at the key a later run would Stat and trust as a complete
-// blob. List and ListStream skip these files, so a leftover from an
-// interrupted write is never listed or trusted as a blob; it is otherwise
-// harmless and is overwritten when the same key is written again.
+// blob. The rclone backend's upload does the same on remotes with a
+// server-side move. List and ListStream skip these files, so a leftover from
+// an interrupted write is never listed or trusted as a blob; it is otherwise
+// harmless.
 const tempSuffix = ".partial"
 
 // Put stores data at the specified key.