0.21.3

• Add support for worktreeconfig extension.
  (Jelmer Vernooij)

• Deprecate Commit.extra; the Git project specifically
  discourages adding custom lines, and the contents of
  Commit.extra are unpredictable as contents
  may be different between different versions of Dulwich
  with support for different headers.

  Commit._extra still exists.
  (Jelmer Vernooij)