py3: have bytes version of os.getenv
os.getenv() on python 3 deals with unicodes. If we want to pass bytes. we have
os.getenvb() which deals with bytes. This patch adds up a pycompat.osgetenv
which deals with bytes on both python 2 and 3.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.