py3: use email.generator.BytesGenerator in patch.split()

This fixes test-import.t on python3.

We add Generator alias in mail module to handle python2/python3
compatibility.

mercurial/mail.py

@@ -9,6 +9,7 b' from __future__ import absolute_import'
 
 import email
 import email.charset
+import email.generator
 import email.header
 import email.message
 import email.parser
@@ -420,6 +421,8 b' def mimeencode(ui, s, charsets=None, dis'
 
 if pycompat.ispy3:
 
+    Generator = email.generator.BytesGenerator
+
     def parse(fp):
         ep = email.parser.Parser()
         # disable the "universal newlines" mode, which isn't binary safe.
@@ -436,6 +439,8 b' if pycompat.ispy3:'
 
 else:
 
+    Generator = email.generator.Generator
+
     def parse(fp):
         ep = email.parser.Parser()
         return ep.parse(fp)

mercurial/patch.py

@@ -11,7 +11,6 b' from __future__ import absolute_import, '
 import collections
 import contextlib
 import copy
-import email
 import errno
 import hashlib
 import os
@@ -107,7 +106,7 b' def split(stream):'
     def mimesplit(stream, cur):
         def msgfp(m):
             fp = stringio()
-            g = email.Generator.Generator(fp, mangle_from_=False)
+            g = mail.Generator(fp, mangle_from_=False)
             g.flatten(m)
             fp.seek(0)
             return fp