merge: do not create a signed tag merge under --ff-only option

Starting at release v1.7.9, if you ask to merge a signed tag, "git merge"
always creates a merge commit, even when the tag points at a commit that
happens to be a descendant of your current commit.

Unfortunately, this interacts rather badly for people who use --ff-only to
make sure that their branch is free of local developments. It used to be
possible to say:

	$ git checkout -b frotz v1.7.9~30
        $ git merge --ff-only v1.7.9

and expect that the resulting tip of frotz branch matches v1.7.9^0 (aka
the commit tagged as v1.7.9), but this fails with the updated Git with:

	fatal: Not possible to fast-forward, aborting.

because a merge that merges v1.7.9 tag to v1.7.9~30 cannot be created by
fast forwarding.

We could teach users that now they have to do

	$ git merge --ff-only v1.7.9^0

but it is far more pleasant for users if we DWIMmed this ourselves.

When an integrator pulls in a topic from a lieutenant via a signed tag,
even when the work done by the lieutenant happens to fast-forward, the
integrator wants to have a merge record, so the integrator will not be
asking for --ff-only when running "git pull" in such a case. Therefore,
this change should not regress the support for the use case v1.7.9 wanted
to add.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

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