This is similar to using lazygit's Git.Paging.ExternalDiffCommand config, except
that the command is configured in git. This can be done either with git's
`diff.external` config, or through .gitattributes, so it gives a bit more
flexibility.
Many people don't understand what this means, which is apparent from the amount
of issues that got filed because of this. Let's get rid of it to avoid this
confusion. People will have to configure their pager twice if they want to use
it both on the command line and in lazygit, which I think is not a big deal.
