When we abort a rebase, we return to the original value of
HEAD. Failing to write a reflog entry means we create a
gap in the reflog (which can cause "git show
HEAD@{5.minutes.ago}" to issue a warning). Plus having the
extra entry makes the reflog easier to follow for a human.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>