[PATCH v3 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. Example usuage: git commit --fixup=reword:<commit>

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

Changes from v2 :

* doc/git-commit.c: reword and simplify the `--fixup=[amend|reword]`
* error out "foo" in the output message when `--fixup=reword:` combined
  with pathspec i.e
  $ git commit --fixup=reword:HEAD~ -- foo
  (I am bit unsure about this as I have directly used the "argv" to get the
  first foo (file) passed. I am doubtful and not sure about, if we can
  use the `pathspec_from_file` in (builtin/commit.c) ? )
* fixes the TRANSLATOR, commit message and other minor cleanup (v2 2/6)

Charvi Mendiratta (6):
  sequencer: export 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                          | 125 ++++++++++++++++++--
 commit.c                                  |  14 +++
 commit.h                                  |   3 +
 sequencer.c                               |  14 ---
 t/t3437-rebase-fixup-options.sh           |  30 +----
 t/t7500-commit-template-squash-signoff.sh | 135 ++++++++++++++++++++++
 8 files changed, 316 insertions(+), 66 deletions(-)

-- 2.29.0.rc1