merge: don't auto-pick destination with `hg merge 'wdir()'` 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. Differential Revision: https://phab.mercurial-scm.org/D7996