Merge branch 'rs/pretty'

Merge branch 'rs/pretty'

* rs/pretty:
  Fix preprocessor logic that determines the availablity of strchrnul().
  Simplify strchrnul() compat code
  --format=pretty: avoid calculating expensive expansions twice
  add strbuf_adddup()
  --pretty=format: parse commit message only once
  --pretty=format: on-demand format expansion
  Add strchrnul()