Merge branch 'jc/rev-list-simplify-merges-first-parent'

Merge branch 'jc/rev-list-simplify-merges-first-parent'

When "--simplify-merges/by-decoration" is given together with
"--first-parent" to "git log", the combination of these options
makes the simplification logic to use in-core commit objects that
haven't been examined for relevance, either producing incorrect
result or taking too long to produce any output.  Teach the
simplification logic to ignore commits that the first-parent
traversal logic ignored when both are in effect to work around the
issue.