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

mercurial/chgserver.py

@@ -108,8 +108,13 @@ def _confighash(ui):
     for section, item in _configsectionitems:
         sectionitems.append(ui.config(section, item))
     sectionhash = _hashlist(sectionitems)
+    # If $CHGHG is set, the change to $HG should not trigger a new chg server
+    if 'CHGHG' in encoding.environ:
+        ignored = {'HG'}
+    else:
+        ignored = set()
     envitems = [(k, v) for k, v in encoding.environ.iteritems()
-                if _envre.match(k)]
+                if _envre.match(k) and k not in ignored]
     envhash = _hashlist(sorted(envitems))
     return sectionhash[:6] + envhash[:6]