receive-pack: drop "n/a" on unpacker errors

The output from git push currently looks like this:

  $ git push dest HEAD
  fatal: [some message from index-pack]
  error: unpack failed: index-pack abnormal exit
  To dest
   ! [remote rejected] HEAD -> master (n/a (unpacker error))

That n/a is meant to be "the per-ref status is not
available" but the nested parentheses just make it look
ugly. Let's turn the final line into just:

   ! [remote rejected] HEAD -> master (unpacker error)

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/receive-pack.c

@@ -701,7 +701,7 @@ static void execute_commands(struct command *commands, const char *unpacker_erro
 
 	if (unpacker_error) {
 		for (cmd = commands; cmd; cmd = cmd->next)
-			cmd->error_string = "n/a (unpacker error)";
+			cmd->error_string = "unpacker error";
 		return;
 	}
 

t/t5504-fetch-receive-strict.sh

@@ -89,7 +89,7 @@ test_expect_success 'push with !receive.fsckobjects' '
 
 cat >exp <<EOF
 To dst
-!	refs/heads/master:refs/heads/test	[remote rejected] (n/a (unpacker error))
+!	refs/heads/master:refs/heads/test	[remote rejected] (unpacker error)
 EOF
 
 test_expect_success 'push with receive.fsckobjects' '