release:prepare should check for remote/local being in sync first

### New feature, improvement proposal

Currently release:prepare only breaks in case of sync issues between remote/local once it tries to push at the very end. This check should be performed first to fail faster in that case (before any modifications to local working directory have been applied).
Although there is a slight chance that between checking and pushing there are additional commits being added this should capture non-pulled changes for most of the cases.

Start by locating the implementation and tests for the release:prepare goal, then trace where remote/local synchronization is currently detected during the release flow. The change is complete when an out-of-sync repository fails before local working-directory modifications, with tests covering that early failure.

Written by the indexing model from the issue text.