"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