[GSoC][PATCH v8 00/20] rebase -i: rewrite in C

This patch series rewrite the interactive rebase from shell to C.

The v7 was based on ffc6fa0e39 ("Fourth batch for 2.19 cycle", 2018-07-24), but this series is based on fe8321ec05 ("Second batch post 2.19", 2017-09-24) due to some conflicts.

Changes since v7:

 - [17/20] The optionnal parameter of `-S' was mandatory in
   rebase--builtin2, making it abort when a signing key is provided to
   `user.signingkey' and `commit.gpgsign' is set to true.  First
   reported on github[0], fixed by Johannes Schindelin.

 - [2, 5, 19, 20/20] Changes due to the rebase.

 - [9, 11/20] Conflict solving.

Alban Gruin (20):
  sequencer: make three functions and an enum from sequencer.c public
  rebase -i: rewrite append_todo_help() in C
  editor: add a function to launch the sequence editor
  rebase -i: rewrite the edit-todo functionality in C
  sequencer: add a new function to silence a command, except if it fails
  rebase -i: rewrite setup_reflog_action() in C
  rebase -i: rewrite checkout_onto() in C
  sequencer: refactor append_todo_help() to write its message to a
    buffer
  sequencer: change the way skip_unnecessary_picks() returns its result
  t3404: todo list with commented-out commands only aborts
  rebase -i: rewrite complete_action() in C
  rebase -i: remove unused modes and functions
  rebase -i: implement the logic to initialize $revisions in C
  rebase -i: rewrite the rest of init_revisions_and_shortrevisions() in
    C
  rebase -i: rewrite write_basic_state() in C
  rebase -i: rewrite init_basic_state() in C
  rebase -i: implement the main part of interactive rebase as a builtin
  rebase--interactive2: rewrite the submodes of interactive rebase in C
  rebase -i: remove git-rebase--interactive.sh
  rebase -i: move rebase--helper modes to rebase--interactive

 .gitignore                     |   1 -
 Makefile                       |   5 +-
 builtin.h                      |   2 +-
 builtin/rebase--helper.c       |  88 ---------
 builtin/rebase--interactive.c  | 271 ++++++++++++++++++++++++++++
 cache.h                        |   1 +
 editor.c                       |  27 ++-
 git-rebase--interactive.sh     | 283 -----------------------------
 git-rebase--preserve-merges.sh |  10 +-
 git-rebase.sh                  |  47 ++++-
 git.c                          |   2 +-
 rebase-interactive.c           |  90 ++++++++++
 rebase-interactive.h           |   8 +
 sequencer.c                    | 320 +++++++++++++++++++++++++++------
 sequencer.h                    |  22 ++-
 strbuf.h                       |   2 +
 t/t3404-rebase-interactive.sh  |  10 ++
 17 files changed, 740 insertions(+), 449 deletions(-)
 delete mode 100644 builtin/rebase--helper.c
 create mode 100644 builtin/rebase--interactive.c
 delete mode 100644 git-rebase--interactive.sh
 create mode 100644 rebase-interactive.c
 create mode 100644 rebase-interactive.h