Merge branch 'jk/diff-highlight-graph-fix'

"diff-highlight" filter (in contrib/) learned to undertand "git log
--graph" output better.

* jk/diff-highlight-graph-fix:
  diff-highlight: detect --graph by indent
  diff-highlight: use flush() helper consistently
  diff-highlight: test graphs with --color
  diff-highlight: test interleaved parallel lines of history
  diff-highlight: prefer "echo" to "cat" in tests
  diff-highlight: use test_tick in graph test
  diff-highlight: correct test graph diagram