rebase: cleanup reset_head() calls

rebase: cleanup reset_head() calls

If ORIG_HEAD is not set by passing RESET_ORIG_HEAD then there is no
need to pass anything for reflog_orig_head. In addition to the callers
fixed in this commit move_to_original_branch() also passes
reflog_orig_head without setting ORIG_HEAD. That caller is mistakenly
passing the message it wants to put in the branch reflog which is not
currently possible so we delay fixing that caller until we can pass
the message as the branch reflog.

A later commit will make it a BUG() to pass reflog_orig_head without
RESET_ORIG_HEAD, that changes cannot be done here as it needs to wait
for move_to_original_branch() to be fixed first.

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