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>

server-info.c

@@ -246,7 +246,7 @@ int update_server_info(int force)
 	errs = errs | update_info_packs(force);
 
 	/* remove leftover rev-cache file if there is any */
-	unlink(git_path("info/rev-cache"));
+	unlink_or_warn(git_path("info/rev-cache"));
 
 	return errs;
 }