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!
``` `` ``` is [deprecated](http://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xcu_chap02.html#tag_23_02_06_03):
> Because of these inconsistent behaviors, the backquoted variety of command substitution is not recommended for new applications that nest command substitutions or attempt to embed complex scripts.