Merge branch 'bh/diff-highlight-graph' into maint

"diff-highlight" script (in contrib/) learned to work better with
"git log -p --graph" output.

* bh/diff-highlight-graph:
  diff-highlight: avoid highlighting combined diffs
  diff-highlight: add multi-byte tests
  diff-highlight: ignore test cruft
  diff-highlight: add support for --graph output
  diff-highlight: add failing test for handling --graph output
  diff-highlight: add some tests