TL;DR: I wrote a bash script that walks through your git repository history and calculates number of commits, number of merged pull requests, number of files and total number of lines of code, broken down per month.
If you want to delete your local branches that have already been merged to master remotely, run this in a bash shell:
$ git branch -D `git branch --merged | grep -v '*' | xargs`
This deletes all branches that are merged, except the currently checked out branch (that’s the part with the start). So you should better run this while you’ve got the master branch checked out.