Fix bug in -z handling of trailing NUL in input

The obvious way to fix this would be to adapt lines to split on null.

However, it's actually nontrivial to rewrite lines. In particular it has a
weird implementation to avoid a space leak. See:
https://gitlab.haskell.org/ghc/ghc/-/issues/4334

Also, while that is a small amount of code, it's covered by a rather
complex copyright and I'd have to include that copyright in git-annex.

So, I opted to filter out the trailing empty string instead.

Sponsored-by: Dartmouth College's Datalad project

CmdLine/Batch.hs

@@ -115,7 +115,17 @@ batchLines (BatchFormat sep _) = do
   where
 	splitter = case sep of
 		BatchLine -> lines
-		BatchNull -> splitc '\0'
+		BatchNull -> elimemptyend . splitc '\0'
+
+	-- When there is a trailing null on the input, eliminate the empty
+	-- string that splitc generates. Other empty strings elsewhere in
+	-- the list are preserved. This is the same effect as how `lines`
+	-- handles a trailing newline.
+	elimemptyend [] = []
+	elimemptyend (x:[])
+		| null x = []
+		| otherwise = [x]
+	elimemptyend (x:rest) = x : elimemptyend rest
 
 -- When concurrency is enabled at the command line, it is used in batch
 -- mode. But, if it's only set in git config, don't use it, because the