push: specify default-push and default as fallback paths

The next patch will remove the "default-push" hack from ui.paths so that
ui.paths["default"].pushurl can be different from "default-push".

mercurial/commands.py

@@ -5663,7 +5663,7 b' def push(ui, repo, dest=None, **opts):'
                 # this lets simultaneous -r, -b options continue working
                 opts.setdefault('rev', []).append("null")
 
-    path = ui.paths.getpath(dest, default='default')
+    path = ui.paths.getpath(dest, default=('default-push', 'default'))
     if not path:
         raise error.Abort(_('default repository not configured!'),
                          hint=_('see the "path" section in "hg help config"'))