Make sure you install it Blender before opening the project in Unity the first time. Otherwise you will get import errors!
First install bfg. If on macOS, you can install it using homebrew
MAKE SURE YOU BACKUP BEFORE MAKING CHANGES
$ bfg --delete-files *.mat.meta
$ bfg --delete-files *.mat
$ bfg --delete-files *.meta
$ git reflog expire --expire=now --all && git gc --prune=now --aggressive