implicit interactive rebase: don't run sequence editor

implicit interactive rebase: don't run sequence editor

If GIT_SEQUENCE_EDITOR is set then rebase runs it when executing
implicit interactive rebases which are supposed to appear
non-interactive to the user. Fix this by setting GIT_SEQUENCE_EDITOR=:
rather than GIT_EDITOR=:. A couple of tests relied on the old behavior
so they are updated to work with the new regime.

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