merge script: handle --no-ff --no-commit correctly

In a --no-ff merge with conflicts, "git commit" used to forget the
--no-ff when used to complete the merge.  That was fixed by
v1.6.1-rc1~134^2 (builtin-commit: use reduce_heads() only when
appropriate, 2008-10-03) for the builtin merge.  Port the change to
the merge script in contrib/examples.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>