This rewrites (the misnamed) setup_reflog_action() from shell to C. The
new version is called prepare_branch_to_be_rebased().
A new command is added to rebase--helper.c, “checkout-base”, as well as
a new flag, “verbose”, to avoid silencing the output of the checkout
operation called by checkout_base_commit().
The function `run_git_checkout()` will also be used in the next commit,
therefore its code is not part of `checkout_base_commit()`.
The shell version is then stripped in favour of a call to the helper.
As $GIT_REFLOG_ACTION is no longer set at the first call of
checkout_onto(), a call to comment_for_reflog() is added at the
beginning of this function.
Signed-off-by: Alban Gruin <alban.gruin@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>