[GSoC] [PATCH v5 0/3] 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 v4:

  -  Remove the `do_reset()` refactored function from sequencer.
     In other words `sequencer: refactor the code to detach HEAD to checkout.c`
     patch was dropped to introduce a new function `reset_hard()` for `rebase.c`
     (as suggested by Johannes).

  -  Fix a case of leak in `rebase: start implementing it as a builtin`.
     (as pointed out by Andrei Rybak and Eric Sunshine).

  -  Wrap the user visible comments in `_()` and used `BUG()` depending on the
     scenarios (as pointed out by Duy Nguyen).

  -  Fix the macro `GIT_PATH_FUNC` which expands to function definition and
     doesn't require semicolons (as pointed out by Beat Bolli).

Pratik Karki (3):
  rebase: start implementing it as a builtin
  rebase: refactor common shell functions into their own file
  builtin/rebase: support running "git rebase <upstream>"

 .gitignore                            |   2 +
 Makefile                              |   4 +-
 builtin.h                             |   1 +
 builtin/rebase.c                      | 406 ++++++++++++++++++++++++++
 git-rebase.sh => git-legacy-rebase.sh |  69 +----
 git-rebase--common.sh                 |  68 +++++
 git.c                                 |   6 +
 7 files changed, 488 insertions(+), 68 deletions(-)
 create mode 100644 builtin/rebase.c
 rename git-rebase.sh => git-legacy-rebase.sh (90%)
 create mode 100644 git-rebase--common.sh