[GSoC] [PATCH v2 0/4] rebase: rewrite rebase in C

As a GSoC project, I have been working on the builtin rebase.

The motivation behind the rewrite of rebase i.e. from shell script to C are for following reasons:

1.  Writing shell scripts and getting it to production is much faster
    than doing the equivalent in C but lacks in performance and extra
    workarounds are needed for non-POSIX platforms.

2.  Git for Windows is at loss as the installer size increases due to
    addition of extra dependencies for the shell scripts which are usually
    available in POSIX compliant platforms.

This series of patches serves to demonstrate a minimal builtin rebase which supports running `git rebase <upstream>` and also serves to ask for reviews.

Changes since v1:

  -  Remove the TODO-rebase.sh which shouldn't be merged into `pu`.

  -  Add newline at the end of the file `builtin/rebase.c` in
     `rebase: start implementing it as a builtin`.

  -  Fix unintentional ordering in `git-rebase--common.sh` and fix the commit
     message in `rebase: refactor common shell functions into their own file`.

  -  Fix wrong expression of `argc` in the handle upstream loop, fix wrong
     type casting code, make the condition simple and fix commit message in
     `builtin/rebase: support running "git-rebase <upstream>".

Pratik Karki (4):
  rebase: start implementing it as a builtin
  rebase: refactor common shell functions into their own file
  sequencer: refactor the code to detach HEAD to checkout.c
  builtin/rebase: support running "git rebase <upstream>"

 .gitignore                            |   2 +
 Makefile                              |   4 +-
 builtin.h                             |   1 +
 builtin/rebase.c                      | 282 ++++++++++++++++++++++++++
 checkout.c                            |  64 ++++++
 checkout.h                            |   3 +
 git-rebase.sh => git-legacy-rebase.sh |  62 +-----
 git-rebase--common.sh                 |  61 ++++++
 git.c                                 |   6 +
 sequencer.c                           |  58 +-----
 10 files changed, 428 insertions(+), 115 deletions(-)
 create mode 100644 builtin/rebase.c
 rename git-rebase.sh => git-legacy-rebase.sh (91%)
 create mode 100644 git-rebase--common.sh