diff -r cf59de802883 -r 7b12a2d2eedc mercurial/mail.py
--- a/mercurial/mail.py	Sat Jun 16 17:56:37 2018 +0900
+++ b/mercurial/mail.py	Sat Jun 16 19:31:07 2018 +0900
@@ -11,6 +11,8 @@
 import email.charset
 import email.header
 import email.message
+import email.parser
+import io
 import os
 import smtplib
 import socket
@@ -322,6 +324,23 @@
         s, cs = _encode(ui, s, charsets)
     return mimetextqp(s, 'plain', cs)
 
+if pycompat.ispy3:
+    def parse(fp):
+        ep = email.parser.Parser()
+        # disable the "universal newlines" mode, which isn't binary safe.
+        # I have no idea if ascii/surrogateescape is correct, but that's
+        # what the standard Python email parser does.
+        fp = io.TextIOWrapper(fp, encoding=r'ascii',
+                              errors=r'surrogateescape', newline=chr(10))
+        try:
+            return ep.parse(fp)
+        finally:
+            fp.detach()
+else:
+    def parse(fp):
+        ep = email.parser.Parser()
+        return ep.parse(fp)
+
 def headdecode(s):
     '''Decodes RFC-2047 header'''
     uparts = []