avoid a traceback with hg branch newbranch; hg up

mercurial/commands.py

@@ -2563,7 +2563,10 b' def _lookup(repo, node, branch=None):'
             node = repo.lookup(node)
         else:
             wc = repo.workingctx()
+            try:
             node = repo.branchtags()[wc.branch()]
+            except KeyError:
+                raise util.Abort(_("branch %s not found") % wc.branch())
     return node
 
 def verify(ui, repo):

tests/test-newbranch

@@ -46,3 +46,7 b' hg id'
 hg up -q 1
 hg up -q
 hg id
+hg branch foobar
+hg up
+
+exit 0

tests/test-newbranch.out

@@ -81,3 +81,4 b' 67ec16bde7f1575d523313b9bca000f6a6f12dca'
 % update with no arguments: tipmost revision of the current branch
 bf1bc2f45e83
 4909a3732169 (foo) tip
+abort: branch foobar not found