branch closing: permit closing the default branch

There was no good reason to special case the 'default' branch.
Allow the 'default' branch to be closed if the user wants it.

If you're uncomfortable about mistakenly closing the default branch,
you can always reopen the branch by commiting a "normal" changeset onto
the closed branch.

mercurial/localrepo.py

@@ -852,8 +852,6 b' class localrepository(repo.repository):'
             user = wctx.user()
             text = wctx.description()
 
-            if branchname == 'default' and extra.get('close'):
-                raise util.Abort(_('closing the default branch is invalid'))
             p1, p2 = [p.node() for p in wctx.parents()]
             c1 = self.changelog.read(p1)
             c2 = self.changelog.read(p2)