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

mercurial/commands.py

@@ -4866,8 +4866,7 b' def merge(ui, repo, node=None, **opts):'
 
     if node:
         node = scmutil.revsingle(repo, node).node()
-
-    if not node:
+    else:
         if ui.configbool(b'commands', b'merge.require-rev'):
             raise error.Abort(
                 _(
@@ -4877,6 +4876,9 b' def merge(ui, repo, node=None, **opts):'
             )
         node = repo[destutil.destmerge(repo)].node()
 
+    if node is None:
+        raise error.Abort(_(b'merging with the working copy has no effect'))
+
     if opts.get(b'preview'):
         # find nodes that are ancestors of p2 but not of p1
         p1 = repo.lookup(b'.')

tests/test-merge2.t

@@ -50,4 +50,8 b''
   adding b
   created new head
 
+  $ hg merge 'wdir()'
+  abort: merging with the working copy has no effect
+  [255]
+
   $ cd ..