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>

builtin-fetch-pack.c

@@ -814,7 +814,7 @@ struct ref *fetch_pack(struct fetch_pack_args *my_args,
 		fd = hold_lock_file_for_update(&lock, shallow,
 					       LOCK_DIE_ON_ERROR);
 		if (!write_shallow_commits(fd, 0)) {
-			unlink(shallow);
+			unlink_or_warn(shallow);
 			rollback_lock_file(&lock);
 		} else {
 			commit_lock_file(&lock);