git: Removing sensitive information from a repository

While you can use the 'filter-branch' switch to effectively erase all trace of a file from a repository - there is a much quicker way to do this using BFG Repo-Cleaner.

Firstly grab an up to date copy of the repo with:
git pull https://github.com/user123/project.git master

Remove the file from the current branch:
git rm 'dirtyfile.txt'

Commit the changes to the local repo:
git commit -m "removal"

Push changes to the remote repo:
git push origin master

Download and execute BFG Repo-Cleaner:
cd /tmp
yum install jre-headless
wget http://repo1.maven.org/maven2/com/madgag/bfg/1.12.15/bfg-1.12.15.jar
cd /path/to/git/repo
java -jar /tmp/bfg-1.12.15.jar --delete-files dirtyfile.txt

Purge the reflog with:
git reflog expire --expire=now --all && git gc --prune=now --aggressive

Finally forcefully push changes to the remote repo:
git push origin master