Gitflow how to identify commits for a feature after feature branch deleted

We have been using Gitflow within SourceTree, on Stash for a short while now, having come from TFS previously. All told the move has been positive.

One thing puzzles me though. The Gitflow approach deletes feature branches once completed, pull requested and successfully merged. We work on several feature branches at one time, so later unpicking which commit belonged to which feature can be very tricky, especially when the feature branch has been long running and has included pulls from develop during that lifetime.

One option that some of the team favour is rigidly prefixing all commit comments with the feature ID, but this feels wrong and prone to error.

Any ideas how to easily identify which commits belonged to a given feature once its branch has been deleted? We are merging rather than rebasing once the feature branch pull request is approved, if that is important.

Git Baby is a git and github fan, let's start git clone.