Merge branch 'jk/diff-highlight-more' into next

Various updates to contrib/diff-highlight, including documentation
updates, test improvements, and color configuration handling.

* jk/diff-highlight-more:
  diff-highlight: fetch all config with one process
  diff-highlight: allow module callers to pass in color config
  diff-highlight: test color config
  diff-highlight: use test_decode_color in tests
  t: add matching negative attributes to test_decode_color
  diff-highlight: check diff-highlight exit status in tests
  diff-highlight: drop perl version dependency back to 5.8
  diff-highlight: mention build instructions