patch: do not cache translated messages (API)

Previously the code caches `i18n._` results in module variables. That causes
issues after an encoding change. Instead of invalidating them manually, we
now just recalculate the translated messages every time `filterpatch` gets
called.

This makes test-commit-interactive.t pass regardless of whether chg or
demandimport is used or not.

.. api: `patch.messages` now lives in `patch.getmessages()`.

   Extensions adding new messages should now wrap the `patch.getmessages`
   method instead of changing `patch.messages` directly.

Differential Revision: https://phab.mercurial-scm.org/D959

mercurial/patch.py

@@ -994,7 +994,8 b' class recordhunk(object):'
     def __repr__(self):
         return '<hunk %r@%d>' % (self.filename(), self.fromline)
 
-messages = {
+def getmessages():
+    return {
     'multiple': {
         'discard': _("discard change %d/%d to '%s'?"),
         'record': _("record change %d/%d to '%s'?"),
@@ -1041,6 +1042,8 b' messages = {'
 
 def filterpatch(ui, headers, operation=None):
     """Interactively filter patch chunks into applied-only chunks"""
+    messages = getmessages()
+
     if operation is None:
         operation = 'record'