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()