update: "deprecate" call to 'merge.update' without a destination

Now that all internal callers pre-compute and set a destination at a higher level
it feels like we can kill this API. This will allow us to simplify this
function. However I feel like this is a bit too central and critical to break
now. I'm adding a devel warning to let extension make catch this in the next
cycle.

mercurial/merge.py

@@ -1080,6 +1080,9 b' def update(repo, node, branchmerge, forc'
             pas = [repo[ancestor]]
 
         if node is None:
+            if (repo.ui.configbool('devel', 'all-warnings')
+                    or repo.ui.configbool('devel', 'oldapi')):
+                repo.ui.develwarn('update with no target')
             rev, _mark, _act = destutil.destupdate(repo)
             node = repo[rev].node()