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

When "git log" gets "--simplify-merges/by-decoration" together with
"--first-parent", 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.

* jc/rev-list-simplify-merges-first-parent:
  revision: ignore side parents while running simplify-merges
  revision: note the lack of free() in simplify_merges()
  revision: "simplify" options imply topo-order sort

diff --cc revision.c
Simple merge