replace direct calls to unlink(2) with unlink_or_warn

This helps to notice when something's going wrong, especially on
systems which lock open files.

I used the following criteria when selecting the code for replacement:
- it was already printing a warning for the unlink failures
- it is in a function which already printing something or is
  called from such a function
- it is in a static function, returning void and the function is only
  called from a builtin main function (cmd_)
- it is in a function which handles emergency exit (signal handlers)
- it is in a function which is obvously cleaning up the lockfiles

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ll-merge.c

@@ -219,7 +219,7 @@ static int ll_ext_merge(const struct ll_merge_driver *fn,
 	close(fd);
  bad:
 	for (i = 0; i < 3; i++)
-		unlink(temp[i]);
+		unlink_or_warn(temp[i]);
 	strbuf_release(&cmd);
 	return status;
 }