update: default to tipmost branch if default branch doesn't exist

mercurial/merge.py

@@ -609,6 +609,9 b' def update(repo, node, branchmerge, forc'
             try:
                 node = repo.branchtags()[wc.branch()]
             except KeyError:
+                if wc.branch() == "default": # no default branch!
+                    node = repo.lookup("tip") # update to tip
+                else:
                 raise util.Abort(_("branch %s not found") % wc.branch())
         overwrite = force and not branchmerge
         forcemerge = force and branchmerge