merge: force edit and no-ff mode when merging a tag object

merge: force edit and no-ff mode when merging a tag object

Now that we allow pulling a tag from the remote site to validate the
authenticity, we should give the user the final chance to verify and edit
the merge message. The integrator is expected to leave a meaningful merge
commit log in the history. Disallow fast-forwarding in such a case to
ensure that a merge commit is always recorded.

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