First make sure that you install Java using homebrew - the Oracle installer makes a terrible fscking mess of your machine :(
$ brew tap caskroom/cask
$ brew install brew-cask
$ brew cask install java
Then you can install the bfg using the homebrew recepie
$ brew install bfg
This gives you an alias bfg
to java -jar bfg.jar
.
You can pretty much follow the directions here. In my case I hadn't pushed the repo to github yet so just did a bare checkout from my local version:
$ cd /tmp
$ git clone --mirror ~/Developer/<my_repo>
Cloning into bare repository 'marc.git'...
done.