Nitpicks

• Escape attributes before writing build.nix
• create build directory under $XDG_CACHE_HOME
• disable pruning in git fetch to work around git limitation
• also start nix-shell if no packages have been changed
• add --remote option to support nixpkgs forks in nix-review rev

"Feature parity"

This releases introduces the wip subcommand from nox-review, thanks to @Ma27.
Now nix-review is at least on par with all features that nox-review provides.