chgserver: do not treat HG as sensitive environ when CHGHG is set When `$CHGHG` is set, `$HG` is ignored by the chg client. Removing it from chg's sensitive environment list would avoid starting up servers unnecessarily when `$CHGHG` is the same while `$HG` is different. Differential Revision: https://phab.mercurial-scm.org/D1177