git-icdiff tweaks

1. Fix quoting of arguments - they were being captured inside the quoted string,
and not being passed to the `git difftool` command.
1. Use the user's preferred pager setting (I like `less -FXR -x4`)

Thanks for the tool, it seems like it's going to be really useful!
Oh, I see.  If `core.pager` has a command to run we need to run it.  Hmm.