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()