Clarify dual license status of subprocess.py file.

The author of the file we stole from Python 2.4 distribution, Peter
Astrand <astrand@lysator.liu.se>, OK'ed to add this at the end of the
licensing terms section of the file:

    Use of this file within git is permitted under GPLv2.

Signed-off-by: Junio C Hamano <junkio@cox.net>