v210917

New features in the v210917 release:

* Plugin support for the 'extra' field, contributed by
  @SirIntellegence.

Bug fixes:

* Don't crash if fast-export was asked to export a Mercurial branch to
  Git and a branch of the same name already existed in the Git repo
  but it was not created by fast export. Thanks to Shun-ichi Goto for
  discovering the crash.

* Don't starting incremental conversions from scratch. Tomas Kolda
  discovered that incremental conversions would always start over from
  scratch due to an oversight in the conversion to Python 3.

Documentation fixes:

* Document the `--ignore-unnamed-heads` option. Contributed by Jason
  Winnebeck.

* Avoid confusion about which file is the main entry point to
  fast-export. Contributed by Ray Luo.
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