If the user doesn't specify a commit to merge with, we'll have
node==None in commands.merge(). We'll then try to find a good
commit to merge with. However, if the user, for some strange reason,
runs hg merge 'wdir()', we'll also have node==None and we'll do
that same. That's clearly not the intent, so let's not do that.
It turns out we'd instead crash on that command after this patch, so I
added special handling of it too.