Let TextMate save (on ⌘R) unless document is untitled

This is a subtle change but it means that all but untitled documents are saved by TextMate where previously the command would always be responsible for saving, which meant encoding, callbacks, and similar weren’t respected, and there was a theoretical synchronization problem (TextMate having to get notified that the file changed on disk and notice that it was (now) identical to the modified version in memory).