config: add sanity assert that files are opened as binary

This helps with some debugging in Python 3, and shouldn't hurt
anything in Python 2. The unusual construction using getattr is done
so that StringIO/BytesIO instances can be used as well as real files.

mercurial/config.py

@@ -170,5 +170,8 b' class config(object):'
     def read(self, path, fp=None, sections=None, remap=None):
         if not fp:
             fp = util.posixfile(path, 'rb')
+        assert getattr(fp, 'mode', r'rb') == r'rb', (
+            'config files must be opened in binary mode, got fp=%r mode=%r' % (
+                fp, fp.mode))
         self.parse(path, fp.read(),
                    sections=sections, remap=remap, include=self.read)