rebase -i: rewrite the edit-todo functionality in C

rebase -i: rewrite the edit-todo functionality in C

This rewrites the edit-todo functionality from shell to C.

To achieve that, a new command mode, `edit-todo`, is added, and the
`write-edit-todo` flag is removed, as the shell script does not need to
write the edit todo help message to the todo list anymore.

The shell version is then stripped in favour of a call to the helper.

Signed-off-by: Alban Gruin <alban.gruin@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>