Clean up gitconfig.

This removes some old aliases and fixes the 'aa' and 'ad' aliases so
that git will stop displaying a warning when they are used.
Add '-M' flag to LESS.
Add a couple of colorschemes to vim.
Add easier-to-press window splitting keys to tmux.
Increase SAVEHIST. Allow zle to backspace over insertion-point.
Clean up xinitrc.

This removes some unused code.