.gitconfig: nuke some unused aliases, add log alias based on http://www.jukie.net/bart/blog/pimping-out-git-log

I changed the colors, but otherwise it's the one there with the author
name:

    [alias]
        lg = log --graph --pretty=format:'%Cred%h%Creset -%C(yellow)%d%Creset %s %Cgreen(%cr) %C(bold blue)<%an>%Creset' --abbrev-commit --date=relative