Default to side-by-side with delta if the terminal is wide enough

[Austin Gill]

Requires making the configuration work nicely with git-gl, which seemed difficult.

• Do not use side-by-side in the fzf --preview pane
• Use side-by-side if the terminal is wide enough in the git-gl pager
• Use side-by-side globally if the terminal is wide enough

Perhaps just the last one is enough, because it looks like FZF plays games with the terminal width.