ui: split the `default` arg out of **kwargs for the internal prompt method

This arg was required anyway, based on how it was accessed.  Having it separate
allows it to be typed though, and this will simplify things for the callers- if
a non-None `default` is passed, the return can never be None.  That can be
expressed with `@overload` when the arg can be typed, but that's not possible
when it is rolled up in **kwargs.

The default value is simply copied from the public `prompt()` above it.

mercurial/ui.py

@@ -1786,8 +1786,8 b' class ui:'
         """
         return self._prompt(msg, default=default)
 
-    def _prompt(self, msg, **opts):
-        default = opts['default']
+    def _prompt(self, msg, default=b'y', **opts):
+        opts = {**opts, 'default': default}
         if not self.interactive():
             self._writemsg(self._fmsgout, msg, b' ', type=b'prompt', **opts)
             self._writemsg(