Merge branch 'dl/octopus-graph-bug'

"git log --graph" for an octopus merge is sometimes colored incorrectly, which is demonstrated and documented but not yet fixed. * dl/octopus-graph-bug: t4214: demonstrate octopus graph coloring failure t4214: explicitly list tags in log t4214: generate expect in their own test cases t4214: use test_merge test-lib: let test_merge() perform octopus merges