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.