logtoprocess: define $HG for children processes

So they can compute the hg version for instance.

Differential Revision: https://phab.mercurial-scm.org/D4768

hgext/logtoprocess.py

@@ -40,7 +40,6 b' import subprocess'
 import sys
 
 from mercurial import (
-    encoding,
     pycompat,
 )
 
@@ -128,7 +127,7 b' def uisetup(ui):'
                 optpairs = (
                     ('OPT_{0}'.format(key.upper()), str(value))
                     for key, value in opts.iteritems())
-                env = dict(itertools.chain(encoding.environ.items(),
+                env = dict(itertools.chain(procutil.shellenviron().items(),
                                            msgpairs, optpairs),
                            EVENT=event, HGPID=str(os.getpid()))
                 runshellcommand(script, env)