Merge branch 'jn/eject-fetch-write-commit-graph-out-of-experimental' into master

"fetch.writeCommitGraph" was enabled when "feature.experimental" is
asked for, but it was found to be a bit too risky even for bold
folks in its current shape.  The configuration has been ejected, at
least for now, from the "experimental" feature set.

* jn/eject-fetch-write-commit-graph-out-of-experimental:
  experimental: default to fetch.writeCommitGraph=false