rebase: fix GIT_REFLOG_ACTION regression

rebase: fix GIT_REFLOG_ACTION regression

The scripted version of "rebase" honored the `GIT_REFLOG_ACTION`,
and some automation scripts expected the reflog entries to be
prefixed with "rebase -i", not "rebase", after running "rebase -i".
This regressed in the reimplementation in C.

Fix that, and add a regression test, both with `GIT_REFLOG_ACTION`
set and unset.

Note: the reflog message for "rebase finished" did *not* honor
GIT_REFLOG_ACTION, and as we are very late in the v2.20.0-rcN phase,
we leave that bug for later (as it seems that that bug has been with
us from the very beginning).

Reported by Ian Jackson.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>