rebase --apply: fix reflog

rebase --apply: fix reflog

move_to_original_branch() passes the message intended for the branch
reflog as `orig_head_msg`. Fix this by adding a `branch_msg` member to
struct reset_head_opts and add a regression test.  Note that these
reflog messages do not respect GIT_REFLOG_ACTION. They are not alone
in that and will be fixed in a future series.

The "merge" backend already has tests that check both the branch and
HEAD reflogs.

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