defmodule MyApp.App.Mixfile do
  use Mix.Project

  def project do
    [
      app: :my_app,
      version: "0.0.1-#{sha()}",
      ...

  def sha do
    case System.cmd("git", ["rev-parse", "HEAD"]) do
      {tag, 0} ->
        String.trim(tag)
      _ ->
        "UNKNOWN"
    end
  end