chgserver: more explicit about sensitive environ variables

Environment variables like HGUSER, HGEDITOR, HGEDITFROM should not trigger
a new chgserver. This patch uses a whitelist for environ variables starting
with "HG" to reduce the number of servers.

I have went through `grep -o "[\"']HG[A-Z_0-9]*['\"]" -hR . | sort -u` so
the list should be up-to-date.

mercurial/chgserver.py

@@ -75,7 +75,8 b' def _hashlist(items):'
 # sensitive environment variables affecting confighash
 _envre = re.compile(r'''\A(?:
                     CHGHG
-                    |HG(?:[A-Z].*)?
+                    |HG(?:DEMANDIMPORT|EMITWARNINGS|MODULEPOLICY|PROF|RCPATH)?
+                    |HG(?:ENCODING|PLAIN).*
                     |LANG(?:UAGE)?
                     |LC_.*
                     |LD_.*