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-remote.c

@@ -525,8 +525,8 @@ static int migrate_file(struct remote *remote)
 		path = git_path("remotes/%s", remote->name);
 	else if (remote->origin == REMOTE_BRANCHES)
 		path = git_path("branches/%s", remote->name);
-	if (path && unlink(path))
-		warning("failed to remove '%s'", path);
+	if (path)
+		unlink_or_warn(path);
 	return 0;
 }