made log_revision log the revision of the configured branch, not master

There are a few factors that made this go unnoticed until now:
1) previous_revision is set but never read
2) current_revision is set but only read when logging the revision to a file on the server that is rarely, if ever, looked at
3) [cap-deploy-tagger](https://github.com/forward3d/cap-deploy-tagger) behaves incorrectly--it really should `fetch(:current_revision)`, but does not.  I've submitted a [PR there](https://github.com/forward3d/cap-deploy-tagger/pull/1) too.