Merge branch 'cm/rebase-i-fixup-amend-reword'

"git commit --fixup=<commit>", which was to tweak the changes made
to the contents while keeping the original log message intact,
learned "--fixup=(amend|reword):<commit>", that can be used to
tweak both the message and the contents, and only the message,
respectively.

* cm/rebase-i-fixup-amend-reword:
  doc/git-commit: add documentation for fixup=[amend|reword] options
  t3437: use --fixup with options to create amend! commit
  t7500: add tests for --fixup=[amend|reword] options
  commit: add a reword suboption to --fixup
  commit: add amend suboption to --fixup to create amend! commit
  sequencer: export and rename subject_length()

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