git-notify: Be careful with "--no-merge"

The "-X" option (which asks git-notify to not report merge commits) was
implemented by setting the "--no-merge" option on each invocation of
git-rev-list(1).  However, we do not only use git-rev-list(1) to get the
list of new commits, but also to check whether the old branch head (or
tag) is a parent of the new branch head (or tag).  For this latter
check, the "--no-merge" option should not be set; otherwise, git-notify
would be fooled to believe that the branch has been rewritten if the old
head was a merge commit.