add a simple nested buffering scheme to ui

mercurial/ui.py

@@ -29,6 +29,7 b' class ui(object):'
                  interactive=True, traceback=False, report_untrusted=True,
                  parentui=None):
         self.overlay = None
+        self.buffers = []
         if parentui is None:
             # this is the parent of all ui children
             self.parentui = None
@@ -367,7 +368,16 b' class ui(object):'
             path = self.config("paths", default)
         return path or loc
 
+    def pushbuffer(self):
+        self.buffers.append([])
+
+    def popbuffer(self):
+        return "".join(self.buffers.pop())
+
     def write(self, *args):
+        if self.buffers:
+            self.buffers[-1].extend([str(a) for a in args])
+        else:
         for a in args:
             sys.stdout.write(str(a))