Digger v2.2 (2015-06-05)

• Fix digger install to work with object.d
• Improve resilience of digger install
• Add --bootstrap switch to build compiler entirely from C++ source
• Replace usage of git bisect run with internal implementation
  • Bisection now prefers cached builds when choosing a commit to test
• Allow customizing the set of components to build during bisection
• Use git plumbing in git cache driver for concurrency and better performance
• Don't cache build failures if the error is likely temporary