chg: ignore HG_* in confighash

The environment variables `HG_*` are usually used by hooks. Unlike `HGPLAIN`
etc, they do not actually affect hg's behavior. So do not include them in
confighash.

This would avoid spawning an unbound number of chg server processes if
commit hook calls hg frequently.

mercurial/chgserver.py

@@ -77,7 +77,7 @@ _configsections = [
 # sensitive environment variables affecting confighash
 _envre = re.compile(r'''\A(?:
                     CHGHG
-                    |HG.*
+                    |HG(?:[A-Z].*)?
                     |LANG(?:UAGE)?
                     |LC_.*
                     |LD_.*