transport-helper.c: do not overwrite forced bit

If the the transport helper says it was a forced update, then it is
a forced update.  It is however possible that an update is forced
without the transport-helper knowing about it, namely because some
higher up code had objections to the update and needed forcing in
order to let it through to the transport helper.  In other words, it
does not necessarily mean the update was *not* forced, when the
helper did not say "forced update".

Signed-off-by: Max Horn <max@quendi.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>