[PATCH v4 0/6][Outreachy] commit: Implementation of "amend!" commit

This patch series teaches `git commit --fixup` to create "amend!" commit
as an alternative that works with `git rebase --autosquash`. It allows to
fixup both the content and the commit message of the specified commit.
Here we add two suboptions to the `--fixup`, first `amend` suboption that
creates an "amend!" commit. It takes the staged changes and also allows to
edit the commit message of the commit we are fixing.
Example usuage:
git commit --fixup=amend:<commit>

Secondly, `reword` suboption that creates an empty "amend!" commit i.e it
ignores the staged changes and only allows to reword/edit the commit message
of the commit we are fixing. `--fixup=reword:<commit>` is a short-hand of
`--fixup=amend:<commit> --only`.
Example usuage:
git commit --fixup=reword:<commit>

** This work is rebased on the top of cm/rebase-i-updates.

Changes from v3 :
(Thanks to Junio C Hamano and Eric Sunshine for the reviews and suggesting the
improvements required in the previous patches )

* Now `--fixup=reword:` aliases `--fixup=amend: --only`. Added the respective
test and updated documentation.
* Modified prepared_amend_commit() in commit.c.
* Renamed the subject_length() to commit_subject_length() [v3 - 1/6]
* Modified the --fixup=[amend|reword] documentation
* Also, updated the commit messages and some other minor cleanup.

Charvi Mendiratta (6):
  sequencer: export and rename subject_length()
  commit: add amend suboption to --fixup to create amend! commit
  commit: add a reword suboption to --fixup
  t7500: add tests for --fixup=[amend|reword] options
  t3437: use --fixup with options to create amend! commit
  doc/git-commit: add documentation for fixup=[amend|reword] options

 Documentation/git-commit.txt              |  40 ++++++-
 Documentation/git-rebase.txt              |  21 ++--
 builtin/commit.c                          | 118 +++++++++++++++++--
 commit.c                                  |  14 +++
 commit.h                                  |   3 +
 sequencer.c                               |  16 +--
 t/t3437-rebase-fixup-options.sh           |  30 +----
 t/t7500-commit-template-squash-signoff.sh | 135 ++++++++++++++++++++++
 8 files changed, 309 insertions(+), 68 deletions(-)

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2.29.0.rc1