Merge branch 'ps/commit-graph-less-paranoid'

Earlier we stopped relying on commit-graph that (still) records
information about commits that are lost from the object store,
which has negative performance implications.  The default has been
flipped to disable this pessimization.

* ps/commit-graph-less-paranoid:
  commit-graph: disable GIT_COMMIT_GRAPH_PARANOIA by default

diff --cc commit-graph.c
Simple merge

diff --cc t/t5318-commit-graph.sh
Simple merge