merge: do not launch an editor on "--no-edit $tag"

merge: do not launch an editor on "--no-edit $tag"

When the user explicitly asked us not to, don't launch an editor.

But do everything else the same way as the "edit" case, i.e. leave the
comment with verification result in the log template and record the
mergesig in the resulting merge commit for later inspection.

Based on initiail analysis by Jonathan Nieder.

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