sequencer: honor GIT_REFLOG_ACTION

sequencer: honor GIT_REFLOG_ACTION

There is a lot of code to honor GIT_REFLOG_ACTION throughout git,
including some in sequencer.c; unfortunately, reflog_message() and its
callers ignored it.  Instruct reflog_message() to check the existing
environment variable, and use it when present as an override to
action_name().

Also restructure pick_commits() to only temporarily modify
GIT_REFLOG_ACTION for a short duration and then restore the old value,
so that when we do this setting within a loop we do not keep adding "
(pick)" substrings and end up with a reflog message of the form
    rebase (pick) (pick) (pick) (finish): returning to refs/heads/master

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>