avoid watchFileSize running backward

This is groundwork for using watchFileSize for downloads from external
special remotes.

In Annex.Content.downloadUrl, this potentially avoids jitter in the
progress meter. When downloading with conduit, the meter gets updated based
on both the size of the file, and on the data flowing through conduit.
If that has not yet been flushed to the file, it seems possible for the
meter to run backwards when meter is updated with the file size.
It's probably only a few kb of jitter, so may not be visible.

Sponsored-by: Dartmouth College's DANDI project

Annex/CopyFile.hs

@@ -57,7 +57,7 @@ tryCopyCoW (CopyCoWTried copycowtried) src dest meterupdate =
 			)
 		)
   where
-	docopycow = watchFileSize dest meterupdate $
+	docopycow = watchFileSize dest meterupdate $ const $
 		copyCoW CopyTimeStamps src dest
 	
 	dest' = toRawFilePath dest