Beef up trackGit

[Chris Warburton]

trackGit adds a repo to chriswarbo.net/git and ~/Programming/repos. We
should have it do a little more.

At the very least we should generate and backup an IPNS key. We already
have the repo name too!

Other things we *might* want:

 - A default.nix file: if not found, ask if we want one; if found, skip.
 - A release.nix file: if not found, ask if we want one; if found, skip.
 - A pkgs.nix file: if not found, ask if we want one; if found, skip.
  - If we want a pkgs.nix, we should ask whether we want nix-config.
  - If we want a pkgs.nix, we should ask which nixpkgs revision to pin.
 - ASV benchmarks: if not found, ask if we want some; if found, skip.

The default.nix, release.nix and pkgs.nix files should be copied from
templates, e.g. stored in ~/.templates.

A decent default.nix template would be:

    {
      packageOnly ? true,
      pkgs        ? import ./pkgs.nix,
      /* OTHER DEPENDENCIES GO HERE*/
    }:
    with builtins;
    with pkgs;
    with { defs = rec {
      package = abort "Replace this with your main package derivation";
    }; };
    if packageOnly
       then defs.package
       else defs

A release.nix template might be:

    {
        stable = import ./. { packageOnly = false; };
      unstable = import ./. { packageOnly = false;
                              pkgs = import <nixpkgs> {}; };
    }

If we ask for ASV benchmarks, we should make a benchmarks/ dir, a
skeleton benchmarks/default.nix which takes root and dir, imports
pkgs.nix from dir and wraps python with an empty paths and vars.

We should also populate a skeletal asv.conf.json with reasonable
defaults:

 - asv-nix plugin, environment, etc.
 - Project name should be repo name
 - Repo can be chriswarbo.net/git/....git
 - Builders and matrix should be included for 'dir'
 - Installer should import benchmarks/ dir

In the future we may add a declarative Hydra jobset, with a job to build
the release.nix and a job which imports that built release.nix