rebase --merge: fix reflog when continuing

rebase --merge: fix reflog when continuing

The reflog message for a conflict resolution committed by "rebase
--continue" looks like

	rebase (continue): commit subject line

Unfortunately the reflog message each subsequent pick look like

	rebase (continue) (pick): commit subject line

Fix this by setting the reflog message for "rebase --continue" in
sequencer_continue() so it does not affect subsequent commits. This
introduces a memory leak similar to the one leaking GIT_REFLOG_ACTION
in pick_commits(). Both of these will be fixed in a future series that
stops the sequencer calling setenv().

If we fail to commit the staged changes then we error out so
GIT_REFLOG_ACTION does not need to be reset in that case.

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