The end-user facing Porcelain level commands like "diff" and "log"
now enables the rename detection by default.

* mm/diff-renames-default:
  diff: activate diff.renames by default
  log: introduce init_log_defaults()
  t: add tests for diff.renames (true/false/unset)
  t4001-diff-rename: wrap file creations in a test
  Documentation/diff-config: fix description of diff.renames