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>