[git] Set editor to '/usr/bin/vim'

- Something in the .vimrc is causing it to exit with a nonzero
  exit status; this 'fix' is from
http://tooky.co.uk/there-was-a-problem-with-the-editor-vi-git-on-mac-os-x/
[git] Remove rules for older git versions
[git] Remove unnecessary diffing-related rules