reset_head(): take struct rebase_head_opts

reset_head(): take struct rebase_head_opts

This function takes a confusingly large number of parameters which
makes it difficult to remember which order to pass them in. The
following commits will add a couple more parameters which makes the
problem worse. To address this change the function to take a struct of
options. Using a struct means that it is no longer necessary to
remember which order to pass the parameters in and anyone reading the
code can easily see which value is passed to each parameter.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>