Don't access repo directly

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For automated environments (like my continuous deployment Jenkins setup) I don't have a git repo, so going through the repo directly to get list of changes between deploys is error-prone. Now, as pointed before in capistrano/capistrano#757 I set previous_revision and current_revision variables back in Capistrano 3. I might be able to work on a fix for this particular scenario on Capistrano 2, but I'm not really sure whether I'll have the time to do it. In Capistrano 3, we should prefer to go through the aforementioned variables instead of the repo instance, as it might not exist at all. All the information should be retrieved from the remote and the current state of the local checkout on the deployment server as opposed to the local one. Comments welcome.