local: fix copying from Windows Volume Shadows

For some files the Windows Volume Shadow Service (VSS) advertises the
file size as X in the directory listing but returns a different number
Y on stat-ing the file. If the file is opened and read there are Y
bytes available for reading.

Existing copy tools copy Y bytes rather than X so for consistency
rclone should do the same.

This fixes the problem by stat-ing the file immediately before opening
it. This will also reduce the unnecessary occurrence of "can't copy -
source file is being updated" errors; if the file has finished
changing by the time we come to copy it then we now can copy it
successfully.

See: https://forum.rclone.org/t/consistently-getting-corrupted-on-transfer-sizes-differ-syncing-to-an-smb-share/42218/

backend/local/local.go

@@ -146,6 +146,11 @@ time we:
 - Only checksum the size that stat gave
 - Don't update the stat info for the file
 
+**NB** do not use this flag on a Windows Volume Shadow (VSS). For some
+unknown reason, files in a VSS sometimes show different sizes from the
+directory listing (where the initial stat value comes from on Windows)
+and when stat is called on them directly. Other copy tools always use
+the direct stat value and setting this flag will disable that.
 `,
 			Default:  false,
 			Advanced: true,
@@ -1123,6 +1128,12 @@ func (o *Object) Open(ctx context.Context, options ...fs.OpenOption) (in io.Read
 		}
 	}
 
+	// Update the file info before we start reading
+	err = o.lstat()
+	if err != nil {
+		return nil, err
+	}
+
 	// If not checking updated then limit to current size.  This means if
 	// file is being extended, readers will read a o.Size() bytes rather
 	// than the new size making for a consistent upload.