ui: provide official way to reset internal state per command

This will allow us to clear in-memory password storage per runcommand().

I've updated commandserver to call resetstate() of both ui and repo.ui because
they may have different states in theory.

mercurial/commandserver.py

@@ -229,12 +229,8 b' class server(object):'
             self.repo.ui = self.repo.dirstate._ui = repoui
             self.repo.invalidateall()
 
-        # reset last-print time of progress bar per command
-        # (progbar is singleton, we don't have to do for all uis)
-        if copiedui._progbar:
-            copiedui._progbar.resetstate()
-
         for ui in uis:
+            ui.resetstate()
             # any kind of interaction must use server channels, but chg may
             # replace channels by fully functional tty files. so nontty is
             # enforced only if cin is a channel.

mercurial/ui.py

@@ -138,6 +138,11 b' class ui(object):'
     def copy(self):
         return self.__class__(self)
 
+    def resetstate(self):
+        """Clear internal state that shouldn't persist across commands"""
+        if self._progbar:
+            self._progbar.resetstate()  # reset last-print time of progress bar
+
     def formatter(self, topic, opts):
         return formatter.formatter(self, topic, opts)