Merge branch 'ps/hash-and-ref-format-from-config'

Merge branch 'ps/hash-and-ref-format-from-config'

The default object hash and ref backend format used to be settable
only with explicit command line option to "git init" and
environment variables, but now they can be configured in the user's
global and system wide configuration.

* ps/hash-and-ref-format-from-config:
  setup: make ref storage format configurable via config
  setup: make object format configurable via config
  setup: merge configuration of repository formats
  t0001: delete repositories when object format tests finish
  t0001: exercise initialization with ref formats more thoroughly