rebase -i: rewrite write_basic_state() in C

rebase -i: rewrite write_basic_state() in C

This rewrites write_basic_state() from git-rebase.sh in C.  This is the
first step in the conversion of init_basic_state(), hence the mode in
rebase--helper.c is called INIT_BASIC_STATE.  init_basic_state() will be
converted in the next commit.

The part of read_strategy_opts() that parses the stategy options is
moved to a new function to allow its use in rebase--helper.c.

Finally, the call to write_basic_state() is removed from
git-rebase--interactive.sh, replaced by a call to `--init-basic-state`.

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