posixfile_nt: '+' should allow read access.

This fixes unexpected I/O error on committing many files
due to cset:8a53b39cd402 (r4902 in crew).

mercurial/util_win32.py

@@ -209,7 +209,7 b' class posixfile_nt(object):'
 
     def __init__(self, name, mode='rb'):
         access = 0
-        if 'r' in mode:
+        if 'r' in mode or '+' in mode:
             access |= win32file.GENERIC_READ
         if 'w' in mode or 'a' in mode or '+' in mode:
             access |= win32file.GENERIC_WRITE