Make trackGit ask whether to make ASV benchmarks

[Chris Warburton]

Ask if we want some ASV benchmarks, if not already found. If so we
should make a benchmarks/ dir, a skeleton benchmarks/default.nix which
takes root and dir, imports pkgs.nix from dir, defines 'fixed' and
'measured' as imports of 'dir' and 'root' with 'packageOnly = false',
and returns a wrapped default python binary (see below).

We should ask whether we should use 'mkBin' from nix-config. There
should be three options:

 - No. Use makeWrapper in runCommand.
 - Yes; we can get it from this project (assumes it's in pkgs.nix)
 - Yes; we should fetch and import nix-config in the benchmarks.

Each of these should have a separate template in ~/.templates:

 - The "no" template should use runCommand and makeWrapper, assuming
   that they're in pkgs.nix. 'dir' should be set, with var name
   'measured'. An example of prefixing PATH should be given, but
   commented-out.
 - The "this project" template should use 'mkBin', assuming that our
   import of pkgs.nix will do. An empty paths list should be included,
   and vars should set 'measured' to 'dir'.
 - The "fetch and import" template should define a nix-config variable
   alongside pkgs, which we take mkBin from. Don't use 'with nix-config'
   since that would override whatever we got from pkgs.nix. The import
   of nix-config should use its defaults; if we cared about nixpkgs
   versions we would be getting stuff from pkgs.nix anyway. The same
   paths and vars as above should be used.

We should also populate an asv.conf.json template with reasonable
defaults:

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

We should ask whether to define a default benchmark which tracks open
issues, defaulting to yes. If yes, this should include a template Python
file in benchmarks/ which looks up 'measured' from the environment and
(if it exists), loops through the contents checking whether each one is
closed (e.g. via a simple search for some header string; whatever
artemis uses).