Merge branch 'jn/eoie-ieot'

As the warning message shown by existing versions of Git for
unknown index extensions is a bit too alarming, two new extensions
are held back and not written by default for the upcoming release.

* jn/eoie-ieot:
  index: make index.threads=true enable ieot and eoie
  ieot: default to not writing IEOT section
  eoie: default to not writing EOIE section