[PATCH 2 of 2] py3: document why os.fsencode() can be used to get back bytes argv

[Yuya Nishihara]

# HG changeset patch
# User Yuya Nishihara <yuya at tcha.org>
# Date 1478696769 -32400
#      Wed Nov 09 22:06:09 2016 +0900
# Node ID 29fff4afc30fe44c5a856b2d3092e2ac985bd60a
# Parent  a08b40ca7b6427d1dee9f52c34df9d16af440592
py3: document why os.fsencode() can be used to get back bytes argv

And a possible Windows issue. I'm sad we have to do such ugly hack, but
that's the unicode on Python 3.

diff --git a/mercurial/pycompat.py b/mercurial/pycompat.py
--- a/mercurial/pycompat.py
+++ b/mercurial/pycompat.py
@@ -43,6 +43,14 @@ if ispy3:
     osname = os.name.encode('ascii')
     ospathsep = os.pathsep.encode('ascii')
     ossep = os.sep.encode('ascii')
+
+    # Since Python 3 converts argv to wchar_t type by Py_DecodeLocale() on Unix,
+    # we can use os.fsencode() to get back bytes argv.
+    #
+    # https://hg.python.org/cpython/file/v3.5.1/Programs/python.c#l55
+    #
+    # TODO: On Windows, the native argv is wchar_t, so we'll need a different
+    # workaround to simulate the Python 2 (i.e. ANSI Win32 API) behavior.
     sysargv = list(map(os.fsencode, sys.argv))
 
     def sysstr(s):