feat(git/gitconfig): add settings for diff-so-fancy
feat(git/gitconfig): add settings for diff-so-fancy

Add settings needed to enable `diff-so-fancy`. Make sure that fallbacks
work for `git log`, `git show` and `git diff` when `diff-so-fancy` is
not installed in the system.

shoichiaizawa/dotfiles,shoichiaizawa/dotfiles,shoichiaizawa/dotfiles